Category: medicine

Aspirin () is the genericized trademark for acetylsalicylic acid (ASA), a nonsteroidal anti-inflammatory drug (NSAID) used to reduce pain, fever, and inflammation, and as an antithrombotic. Aspirin is used to treat inflammatory conditions including Kawasaki disease, pericarditis, and rheumatic fever. Aspirin is also used long-term to help prevent further heart attacks, ischaemic strokes, and blood clots in people at high risk. For pain or fever, effects typically begin within 30 minutes. Aspirin works similarly to other NSAIDs but also suppresses the normal functioning of platelets. One common adverse effect is an upset stomach. More significant side effects include stomach ulcers, stomach bleeding, and worsening asthma. Bleeding risk is greater among those who are older, drink alcohol, take other NSAIDs, or are on other blood thinners. Aspirin is not recommended in the last part of pregnancy. It is not generally recommended in children with viral infections because of the risk of Reye syndrome. High doses may result in ringing in the ears. A precursor to aspirin found in the bark of the willow tree (genus Salix), salicin, is metabolized in the human gut into the medicinally active compound salicylic acid and has been used for its health effects for at least 2,400 years. Pharmacology sought a synthetic alternative. In 1853, the chemist Charles Frédéric Gerhardt treated the medicine sodium salicylate with acetyl chloride to produce acetylsalicylic acid for the first time. Over the next 50 years, other chemists, mostly of the German company Bayer, established the chemical structure and devised more efficient production methods. Felix Hoffmann (or perhaps Arthur Eichengrün) of Bayer was the first to produce acetylsalicylic acid in a pure, stable form in 1897. By 1899, Bayer had dubbed this drug Aspirin and was selling it globally. Aspirin is available without medical prescription as a proprietary or generic medication in most jurisdictions. It is one of the most widely used medications globally, with an estimated 40,000 tonnes (44,000 tons) (50 to 120 billion pills) consumed each year, and is on the World Health Organization's List of Essential Medicines. In 2023, it was the 46th most commonly prescribed medication in the United States, with more than 14 million prescriptions.

An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention of such infections. They may either kill or inhibit the growth of bacteria. A limited number of antibiotics also possess antiprotozoal activity. Antibiotics are not effective against viruses such as the ones which cause the common cold or influenza. Drugs which inhibit growth of viruses are termed antiviral drugs or antivirals. Antibiotics are also not effective against fungi. Drugs which inhibit growth of fungi are called antifungal drugs. Sometimes, the term antibiotic—literally "opposing life", from the Greek roots ἀντι anti, "against" and βίος bios, "life"—is broadly used to refer to any substance used against microbes, but in the usual medical usage, antibiotics (such as penicillin) are those produced naturally (by one microorganism fighting another), whereas non-antibiotic antibacterials (such as sulfonamides and antiseptics) are fully synthetic. However, both classes have the same effect of killing or preventing the growth of microorganisms, and both are included in antimicrobial chemotherapy. "Antibacterials" include bactericides, bacteriostatics, antibacterial soaps, and chemical disinfectants, whereas antibiotics are an important class of antibacterials used more specifically in medicine and sometimes in livestock feed.

Cardiology (from Ancient Greek καρδίᾱ (kardiā) 'heart' and -λογία (-logia) 'study') is the study of the heart. Cardiology is a branch of medicine that deals with disorders of the heart and the cardiovascular system, and it is a sub-specialty of internal medicine. The field includes medical diagnosis and treatment of congenital heart defects, coronary artery disease, heart failure, valvular heart disease, and electrophysiology. Physicians who specialize in this field of medicine are called cardiologists. Pediatric cardiologists are pediatricians who specialize in cardiology. Physicians who specialize in cardiac surgery are called cardiothoracic surgeons or cardiac surgeons, a specialty of general surgery.

Chemotherapy (often abbreviated chemo, sometimes CTX and CTx) is the type of cancer treatment that uses one or more anti-cancer drugs (chemotherapeutic agents or alkylating agents) in a standard regimen. Chemotherapy may be given with a curative intent (which almost always involves combinations of drugs), or it may aim only to prolong life or to reduce symptoms (palliative chemotherapy). Chemotherapy is one of the major categories of the medical discipline specifically devoted to pharmacotherapy for cancer, which is called medical oncology. The term chemotherapy now means the non-specific use of intracellular poisons to inhibit mitosis (cell division) or to induce DNA damage (so that DNA repair can augment chemotherapy). This meaning excludes the more-selective agents that block extracellular signals (signal transduction). Therapies with specific molecular or genetic targets, which inhibit growth-promoting signals from classic endocrine hormones (primarily estrogens for breast cancer and androgens for prostate cancer), are now called hormonal therapies. Other inhibitions of growth-signals, such as those associated with receptor tyrosine kinases, are targeted therapy. The use of drugs (whether chemotherapy, hormonal therapy, or targeted therapy) is systemic therapy for cancer: they are introduced into the blood stream (the system) and therefore can treat cancer anywhere in the body. Systemic therapy is often used with other, local therapy (treatments that work only where they are applied), such as radiation, surgery, and hyperthermia. Traditional chemotherapeutic agents are cytotoxic by means of interfering with cell division (mitosis) but cancer cells vary widely in their susceptibility to these agents. To a large extent, chemotherapy can be thought of as a way to damage or stress cells, which may then lead to cell death if apoptosis is initiated. Many of the side effects of chemotherapy can be traced to damage to normal cells that divide rapidly and are thus sensitive to anti-mitotic drugs: cells in the bone marrow, digestive tract and hair follicles. This results in the most common side-effects of chemotherapy: myelosuppression (decreased production of blood cells, hence that also immunosuppression), mucositis (inflammation of the lining of the digestive tract), and alopecia (hair loss). Because of the effect on immune cells (especially lymphocytes), chemotherapy drugs often find use in a host of diseases that result from harmful overactivity of the immune system against self (so-called autoimmunity). These include rheumatoid arthritis, systemic lupus erythematosus, multiple sclerosis, vasculitis, malignant hypertension and many others.

Human anatomy (gr. ἀνατομία, "dissection", from ἀνά, "up", and τέμνειν, "cut") is primarily the scientific study of the morphology of the human body. Anatomy is subdivided into gross anatomy and microscopic anatomy. Gross anatomy (also called macroscopic anatomy, topographical anatomy, regional anatomy, or anthropotomy) is the study of anatomical structures that can be seen by the naked eye. Microscopic anatomy is the study of minute anatomical structures assisted with microscopes, which includes histology (the study of the organization of tissues), and cytology (the study of cells). Anatomy, human physiology (the study of function), and biochemistry (the study of the chemistry of living structures) are complementary basic medical sciences that are generally together (or in tandem) to students studying medical sciences. In some of its facets human anatomy is closely related to embryology, comparative anatomy and comparative embryology, through common roots in evolution; for example, much of the human body maintains the ancient segmental pattern that is present in all vertebrates with basic units being repeated, which is particularly obvious in the vertebral column and in the ribcage, and can be traced from very early embryos. The human body consists of biological systems, that consist of organs, that consist of tissues, that consist of cells and connective tissue. The history of anatomy has been characterized, over a long period of time, by a continually developing understanding of the functions of organs and structures of the body. Methods have also advanced dramatically, advancing from examination of animals through dissection of fresh and preserved cadavers (corpses) to technologically complex techniques developed in the 20th century.

Insulin ( ; from Latin insula 'island') is a peptide hormone produced by beta cells of the pancreatic islets encoded in humans by the insulin (INS) gene. It is the main anabolic hormone of the body. It regulates the metabolism of carbohydrates, fats, and protein by promoting the absorption of glucose from the blood into cells of the liver, fat, and skeletal muscles. In these tissues the absorbed glucose is converted into either glycogen, via glycogenesis, or fats (triglycerides), via lipogenesis; in the liver, glucose is converted into both. Glucose production and secretion by the liver are strongly inhibited by high concentrations of insulin in the blood. Circulating insulin also affects the synthesis of proteins in a wide variety of tissues. It is thus an anabolic hormone, promoting the conversion of small molecules in the blood into large molecules in the cells. Low insulin in the blood has the opposite effect, promoting widespread catabolism, especially of reserve body fat. Beta cells are sensitive to blood sugar levels so that they secrete insulin into the blood in response to high level of glucose, and inhibit secretion of insulin when glucose levels are low. Insulin production is also regulated by glucose: high glucose promotes insulin production while low glucose levels lead to lower production. Insulin enhances glucose uptake and metabolism in the cells, thereby reducing blood sugar. Their neighboring alpha cells, by taking their cues from the beta cells, secrete glucagon into the blood in the opposite manner: increased secretion when blood glucose is low, and decreased secretion when glucose concentrations are high. Glucagon increases blood glucose by stimulating glycogenolysis and gluconeogenesis in the liver. The secretion of insulin and glucagon into the blood in response to the blood glucose concentration is the primary mechanism of glucose homeostasis. Decreased or absent insulin activity results in diabetes, a condition of high blood sugar level (hyperglycaemia). There are two types of the disease. In type 1 diabetes, the beta cells are destroyed by an autoimmune reaction so that insulin can no longer be synthesized or be secreted into the blood. In type 2 diabetes, the destruction of beta cells is less pronounced than in type 1, and is not due to an autoimmune process. Instead, there is an accumulation of amyloid in the pancreatic islets, which likely disrupts their anatomy and physiology. The pathogenesis of type 2 diabetes is not well understood but reduced population of islet beta-cells, reduced secretory function of islet beta-cells that survive, and peripheral tissue insulin resistance are known to be involved. Type 2 diabetes is characterized by increased glucagon secretion which is unaffected by, and unresponsive to the concentration of blood glucose. But insulin is still secreted into the blood in response to the blood glucose. As a result, glucose accumulates in the blood. The human insulin protein is composed of 51 amino acids, and has a molecular mass of 5808 Da. It is a heterodimer of an A-chain and a B-chain, which are linked together by disulfide bonds. Insulin's structure varies slightly between species of animals. Insulin from non-human animal sources differs somewhat in effectiveness (in carbohydrate metabolism effects) from human insulin because of these variations. Porcine insulin is especially close to the human version, and was widely used to treat type 1 diabetics before human insulin could be produced in large quantities by recombinant DNA technologies. Insulin was the first peptide hormone discovered. Frederick Banting and Charles Best, working in the laboratory of John Macleod at the University of Toronto, were the first to isolate insulin from dog pancreas in 1921. Frederick Sanger sequenced the amino acid structure in 1951, which made insulin the first protein to be fully sequenced. The crystal structure of insulin in the solid state was determined by Dorothy Hodgkin in 1969. Insulin is also the first protein to be chemically synthesised and produced by DNA recombinant technology. It is on the WHO Model List of Essential Medicines, the most important medications needed in a basic health system.

Neurology (from Greek: νεῦρον (neûron), "string, nerve" and the suffix -logia, "study of") is the branch of medicine dealing with the diagnosis and treatment of all categories of conditions and disease involving the nervous system, which comprises the brain, the spinal cord and the peripheral nerves. Neurological practice relies heavily on the field of neuroscience, the scientific study of the nervous system, using various techniques of neurotherapy. A neurologist is a physician specializing in neurology and trained to investigate, diagnose and treat neurological disorders. Neurologists diagnose and treat myriad neurologic conditions, including stroke, epilepsy, movement disorders such as Parkinson's disease, brain infections, autoimmune neurologic disorders such as multiple sclerosis, sleep disorders, brain injury, headache disorders like migraine, tumors of the brain and dementias such as Alzheimer's disease. Neurologists may also have roles in clinical research, clinical trials, and basic or translational research. Neurology is a nonsurgical specialty; its corresponding surgical specialty is neurosurgery.

Penicillins (P, PCN or PEN) are a group of β-lactam antibiotics originally obtained from Penicillium moulds, principally P. chrysogenum and P. rubens. Most penicillins in clinical use are synthesised by P. chrysogenum using deep tank fermentation and then purified. A number of natural penicillins have been discovered, but only two purified compounds are in clinical use: penicillin G (intramuscular or intravenous use) and penicillin V (given by mouth). Penicillins were among the first medications to be effective against many bacterial infections caused by staphylococci and streptococci. They are still widely used today for various bacterial infections, though many types of bacteria have developed resistance following extensive use. In the United States, 10% of the population claims penicillin allergies, but because the frequency of positive skin test results decreases by 10% with each year of avoidance, 90% of these patients can eventually tolerate penicillin. Additionally, those with penicillin allergies can usually tolerate cephalosporins (another group of β-lactam) because the immunoglobulin E (IgE) cross-reactivity is only 3%. Penicillin was discovered in 1928 by the Scottish physician Alexander Fleming as a crude extract of P. rubens. Fleming's student Cecil George Paine was the first to successfully use penicillin to treat eye infection (neonatal conjunctivitis) in 1930. The purified compound (penicillin F) was isolated in 1940 by a research team led by Howard Florey and Ernst Boris Chain at the University of Oxford. Fleming first used the purified penicillin to treat streptococcal meningitis in 1942. The 1945 Nobel Prize in Physiology or Medicine was shared by Chain, Fleming and Florey. Several semisynthetic penicillins are effective against a broader spectrum of bacteria: these include the antistaphylococcal penicillins, aminopenicillins, and antipseudomonal penicillins.

Pharmacology is the science of drugs and medications, including a substance's origin, composition and interaction with biological systems; specifically through pharmacokinetics, pharmacodynamics, therapeutic use, and toxicology. The discipline examines these interactions through pharmacokinetics (what the body does to the drug) and pharmacodynamics (what the drug does to the body), both of which determine how a substance alters normal or abnormal biochemical function. Substances with medicinal properties are classified as pharmaceuticals, while the term drug encompasses any chemical agent that alters biological processes. The field encompasses drug composition and properties, functions, sources, medicinal chemistry, drug design, molecular and cellular mechanisms, organ/systems mechanisms, signal transduction/cellular communication, molecular diagnostics, interactions, chemical biology, therapy, medical applications, toxicology, and antipathogenic capabilities. The two main areas of pharmacology are pharmacodynamics and pharmacokinetics. Pharmacodynamics studies the effects of a drug on biological systems, and pharmacokinetics studies the effects of biological systems on a drug. In broad terms, pharmacodynamics discusses the chemicals with biological receptors, and pharmacokinetics discusses the liberation, absorption, distribution, metabolism, and excretion (LADME) of chemicals from the biological systems. Pharmacology is not synonymous with pharmacy, though the two terms are frequently confused. Pharmacology is a branch of medical and biological sciences which encompasses the research, discovery, and characterization of chemicals exhibiting biological effects, alongside the elucidation of cellular and organismal function in relation to these chemicals. In contrast, pharmacy, a health services profession, is concerned with the application of the principles learned from pharmacology, pharmaceutics, medicinal chemistry, pharmacognosy, clinical pharmacy and others in its clinical settings; whether it be in a dispensing or clinical care role. In either field, the primary contrast between the two is their distinction between direct-patient care, pharmacy practice, and the science-oriented research field, driven by pharmacology.

In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can change into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type of cell in a cell lineage. They are found in both embryonic and adult organisms, but they have slightly different properties in each. They are usually distinguished from progenitor cells, which cannot divide indefinitely, and precursor or blast cells, which are usually committed to differentiating into one cell type. In mammals, roughly 50 to 150 cells make up the inner cell mass during the blastocyst stage of embryonic development, around days 5–14. These have stem-cell capability. In vivo, they eventually differentiate into all of the body's cell types (making them pluripotent). This process starts with the differentiation into the three germ layers – the ectoderm, mesoderm and endoderm – at the gastrulation stage. However, when they are isolated and cultured in vitro, they can be kept in the stem-cell stage and are known as embryonic stem cells (ESCs). Adult stem cells are found in a few select locations in the body, known as niches, such as those in the bone marrow or gonads. They exist to replenish rapidly lost cell types and are multipotent or unipotent, meaning they only differentiate into a few cell types or one type of cell. In mammals, they include, among others, hematopoietic stem cells, which replenish blood and immune cells, basal cells, which maintain the skin epithelium, and mesenchymal stem cells, which maintain bone, cartilage, muscle and fat cells. Adult stem cells are a small minority of cells; they are vastly outnumbered by the progenitor cells and terminally differentiated cells that they differentiate into. Research into stem cells grew out of findings by Canadian biologists Ernest McCulloch, James Till and Andrew J. Becker at the University of Toronto and the Ontario Cancer Institute in the 1960s. As of 2016, the only established medical therapy using stem cells is hematopoietic stem cell transplantation, first performed in 1958 by French oncologist Georges Mathé. Since 1998 however, it has been possible to culture and differentiate human embryonic stem cells (in stem-cell lines). The process of isolating these cells has been controversial, because it typically results in the destruction of the embryo. Sources for isolating ESCs have been restricted in some European countries and Canada, but others such as the UK and China have promoted the research. Somatic cell nuclear transfer is a cloning method that can be used to create a cloned embryo for the use of its embryonic stem cells in stem cell therapy. In 2006, a Japanese team led by Shinya Yamanaka discovered a method to convert mature body cells back into stem cells. These were termed induced pluripotent stem cells (iPSCs).

A blood type (also known as a blood group) is a classification of blood based on the presence and absence of antibodies and inherited antigenic substances on the surface of red blood cells (RBCs). These antigens may be proteins, carbohydrates, glycoproteins, or glycolipids, depending on the blood group system. Some of these antigens are also present on the surface of other types of cells of various tissues. Several of these red blood cell surface antigens can stem from one allele (or an alternative version of a gene) and collectively form a blood group system. Blood types are inherited and represent contributions from both parents of an individual. As of June 2025, a total of 48 human blood group systems are recognized by the International Society of Blood Transfusion (ISBT). The two most important blood group systems are ABO and Rh; they determine someone's blood type (A, B, AB, and O, with + or − denoting RhD status) for suitability in blood transfusion.

Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and determinants of health and disease conditions in a defined population, and application of this knowledge to prevent diseases. It is a cornerstone of public health, and shapes policy decisions and evidence-based practice by identifying risk factors for disease and targets for preventive healthcare. Epidemiologists help with study design, collection, and statistical analysis of data, amend interpretation and dissemination of results (including peer review and occasional systematic review). Epidemiology has helped develop methodology used in clinical research, public health studies, and, to a lesser extent, basic research in the biological sciences. Major areas of epidemiological study include disease causation, transmission, outbreak investigation, disease surveillance, environmental epidemiology, forensic epidemiology, occupational epidemiology, screening, biomonitoring, and comparisons of treatment effects such as in clinical trials. Epidemiologists rely on other scientific disciplines like biology to better understand disease processes, statistics to make efficient use of the data and draw appropriate conclusions, social sciences to better understand proximate and distal causes, and engineering for exposure assessment. Epidemiology, literally meaning 'the study of what is upon the people', is derived from Greek epi 'upon, among'; demos 'people' and logos 'study', referring to the study of human populations. However, the term is widely used in studies of zoological populations (veterinary epidemiology), although the term 'epizootiology' was historically used. 'Epidemiology' has also been applied to studies of plant populations (botanical or plant disease epidemiology). The distinction between "epidemic" and "endemic" was first drawn by Hippocrates, The term "epidemiology" appears to have first been used to describe the study of epidemics in 1802 by the Spanish physician Joaquín de Villalba in Epidemiología Española. Epidemiologists also study the interaction of diseases in a population, a condition known as a syndemic. The term epidemiology is now widely applied to cover the description and causation of not only epidemic, infectious disease, but of disease in general, including related conditions and, especially since the 20th century, chronic diseases such as diabetes, cardiovascular disease, and cancer. Some examples of topics examined through epidemiology include high blood pressure, mental illness and obesity. Therefore, this epidemiology is based upon how the pattern of the disease causes change in the function of human beings.

Cancer is a group of diseases involving uncontrolled cell growth typically resulting in tumors with the potential to invade or spread to other parts of the body. These malignant tumors contrast with benign tumors, which do not spread. Over 100 types of cancers affect humans. About 33% of deaths from cancer are caused by tobacco and alcohol, obesity, lack of fruit and vegetables in the diet and lack of exercise. Other factors include certain infections, exposure to ionizing radiation, and environmental pollutants. Infection with specific viruses, bacteria, and parasites causes approximately 16–18% of cancers worldwide. These infectious agents include Helicobacter pylori, hepatitis B, hepatitis C, and HPV. Human immunodeficiency virus (HIV) does not directly cause cancer, but it causes immune deficiency that can increase the risk of cancer from other infections, sometimes up to several thousandfold. Vaccination against the hepatitis B virus and the human papillomavirus have been shown to nearly eliminate the risk of cancers caused by these viruses. These environmental factors act, at least partly, by changing the genes of a cell. Typically, many genetic changes are required before cancer develops. Approximately 5–10% of cancers are due to inherited genetic defects. Cancer can be detected by certain signs and symptoms or screening tests. It is then typically further investigated by medical imaging and confirmed by biopsy. The risk of developing cancer can be reduced by not smoking, maintaining a healthy weight, limiting alcohol intake, eating plenty of vegetables, fruits, and whole grains, vaccination against certain infectious diseases, limiting consumption of processed meat and red meat, and limiting exposure to direct sunlight. Early detection through screening is useful for cervical and colorectal cancer. The benefits of screening for breast cancer are controversial. Cancer is often treated with some combination of radiation therapy, surgery, chemotherapy and targeted therapy. More personalized therapies that harness a patient's immune system are emerging in the field of cancer immunotherapy. Palliative care is a medical specialty that delivers advanced pain and symptom management, which may be particularly important in those with advanced disease. The chance of survival depends on the type of cancer and extent of disease at the start of treatment. In children under 15 at diagnosis, the five-year survival rate in the developed world is on average 80%. For cancer in the United States, the average five-year survival rate is 66% for all ages. In 2015, about 90.5 million people worldwide had cancer. In 2019, annual cancer cases grew by 23.6 million people, and there were 10 million deaths worldwide, representing over the previous decade increases of 26% and 21%, respectively. The most common types of cancer in males are lung cancer, prostate cancer, colorectal cancer, and stomach cancer. In females, the most common types are breast cancer, colorectal cancer, lung cancer, and cervical cancer. If skin cancer other than melanoma were included in total new cancer cases each year, it would account for around 40% of cases. In children, acute lymphoblastic leukemia and brain tumors are most common, except in Africa, where non-Hodgkin lymphoma occurs more often. In 2012, about 165,000 children under 15 years of age were diagnosed with cancer. The risk of cancer increases significantly with age, and many cancers occur more commonly in developed countries. Rates are increasing as more people live to an old age and as lifestyle changes occur in the developing world. The global total economic costs of cancer were estimated at US$1.16 trillion (equivalent to $1.71 trillion in 2025) per year as of 2010.

The germ theory of disease is the currently accepted scientific theory explaining the cause of infectious diseases. It states that microorganisms known as pathogens or "germs" can cause disease. These small organisms, which are too small to be seen without magnification, invade animals, plants, and even bacteria. Their growth and reproduction within their hosts can cause disease. "Germ" refers not just to bacteria but to any type of microorganism, such as protists or fungi, or other pathogens, including parasites, viruses, prions, or viroids. Even when a pathogen is the principal cause of a disease, environmental and hereditary factors often influence the severity of the disease, and whether a potential host individual becomes infected when exposed to the pathogen. Pathogens are disease-causing agents that can pass from one individual to another, across multiple domains of life. Basic forms of germ theory were proposed by Girolamo Fracastoro in 1546, and expanded upon by Marcus von Plenciz in 1762. However, such views were held in disdain in Europe, where Galen's miasma theory remained dominant among scientists and doctors. By the early 19th century, the first vaccine, smallpox vaccination, was commonplace in Europe, though doctors were unaware of how it worked or how to extend the principle to other diseases. A transitional period began in the late 1850s with the work of Louis Pasteur. This work was later extended by Robert Koch in the 1880s. By the end of that decade, the miasma theory was struggling to compete with the germ theory of disease. Viruses were initially discovered in the 1890s. Eventually, a "golden era" of bacteriology ensued, during which the germ theory quickly led to the identification of the actual organisms that cause many diseases.

The human brain is the central organ of the nervous system, and with the spinal cord, comprises the central nervous system. It consists of the cerebrum, the brainstem and the cerebellum. The brain controls most of the activities of the body, processing, integrating, and coordinating the information it receives from the sensory nervous system. The brain integrates sensory information and coordinates instructions sent to the rest of the body. The cerebrum, the largest part of the human brain, consists of two cerebral hemispheres. Each hemisphere has an inner core composed of white matter, and an outer surface – the cerebral cortex – composed of grey matter. The cortex has an outer layer, the neocortex, and an inner allocortex. The neocortex is made up of six neuronal layers, while the allocortex has three or four. Each hemisphere is divided into four lobes – the frontal, parietal, temporal, and occipital lobes. The frontal lobe is associated with executive functions including self-control, planning, reasoning, and abstract thought, while the occipital lobe is dedicated to vision. Within each lobe, cortical areas are associated with specific functions, such as the sensory, motor, and association regions. Although the left and right hemispheres are broadly similar in shape and function, some functions are associated with one side, such as language in the left and visual-spatial ability in the right. The hemispheres are connected by commissural nerve tracts, the largest being the corpus callosum. The cerebrum is connected by the brainstem to the spinal cord. The brainstem consists of the midbrain, the pons, and the medulla oblongata. The cerebellum is connected to the brainstem by three pairs of nerve tracts called cerebellar peduncles. Within the cerebrum is the ventricular system, consisting of four interconnected ventricles in which cerebrospinal fluid is produced and circulated. Underneath the cerebral cortex are several structures, including the thalamus, the epithalamus, the pineal gland, the hypothalamus, the pituitary gland, and the subthalamus; the limbic structures, including the amygdalae and the hippocampi, the claustrum, the various nuclei of the basal ganglia, the basal forebrain structures, and three circumventricular organs. Brain structures that are not on the midplane exist in pairs; for example, there are two hippocampi and two amygdalae. The cells of the brain include neurons and supportive glial cells. There are more than 86 billion neurons in the brain, and a more or less equal number of other cells. Brain activity is made possible by the interconnections of neurons and their release of neurotransmitters in response to nerve impulses. Neurons connect to form neural pathways, neural circuits, and elaborate network systems. The whole circuitry is driven by the process of neurotransmission. The brain is protected by the skull, suspended in cerebrospinal fluid, and isolated from the bloodstream by the blood–brain barrier. However, the brain is still susceptible to damage, disease, and infection. Damage can be caused by trauma, or a loss of blood supply known as a stroke. The brain is susceptible to degenerative disorders, such as Parkinson's disease, dementias including Alzheimer's disease, and multiple sclerosis. Psychiatric conditions, including schizophrenia and clinical depression, are thought to be associated with brain dysfunctions. The brain can also be the site of tumours, both benign and malignant; these mostly originate from other sites in the body. The study of the anatomy of the brain is neuroanatomy, while the study of its function is neuroscience. Numerous techniques are used to study the brain. Specimens from other animals, which may be examined microscopically, have traditionally provided much information. Medical imaging technologies such as functional neuroimaging, and electroencephalography (EEG) recordings are important in studying the brain. The medical history of people with brain injury has provided insight into the function of each part of the brain. Neuroscience research has expanded considerably, and research is ongoing. In culture, the philosophy of mind has for centuries attempted to address the question of the nature of consciousness and the mind–body problem. The pseudoscience of phrenology attempted to localise personality attributes to regions of the cortex in the 19th century. In science fiction, brain transplants are imagined in tales such as the 1942 Donovan's Brain.

The human immunodeficiency virus (HIV) is a retrovirus that attacks the immune system. Without treatment, it can lead to a spectrum of conditions including acquired immunodeficiency syndrome (AIDS). It is a preventable disease. It can be managed with treatment and become a manageable chronic health condition. While there is no cure or vaccine for HIV, antiretroviral treatment can slow the course of the disease, and, if used before significant disease progression, can extend the life expectancy of someone living with HIV to a nearly standard level. An HIV-positive person on treatment can expect to live a normal life, and die with the virus, not of it. Effective treatment for HIV-positive people (people living with HIV) involves a life-long regimen of medicine to suppress the virus, making the viral load undetectable. Early testing can show if treatment is needed to stop progression and to prevent infecting others. Treatment is recommended as soon as the diagnosis is made. An HIV-positive person who has an undetectable viral load as a result of long-term treatment has effectively no risk of transmitting HIV sexually. Campaigns by UNAIDS and organizations around the world have communicated this as Undetectable = Untransmittable. Without treatment the infection can interfere with the immune system, and eventually progress to AIDS, sometimes taking many years. Following initial infection an individual may not notice any symptoms, or may experience a brief period of influenza-like illness. During this period the person may not know that they are HIV-positive, yet they will be able to pass on the virus. Typically, this period is followed by a prolonged incubation period with no symptoms. Eventually the HIV infection increases the risk of developing other infections such as tuberculosis, as well as other opportunistic infections, and tumors which are rare in people who have normal immune function. The late stage is often also associated with unintended weight loss. Without treatment, the life expectancy of an individual with HIV is around 11 years. HIV is spread primarily by unprotected sex (including anal, oral and vaginal sex), contaminated hypodermic needles or blood transfusions, and from mother to child during pregnancy, delivery, or breastfeeding. Some bodily fluids, such as saliva, sweat, and tears, do not transmit the virus. Oral sex has little risk of transmitting the virus. Ways to avoid catching HIV and preventing the spread include safe sex, treatment to prevent infection ("PrEP"), treatment to stop infection in someone who has been recently exposed ("PEP"), treating those who are infected, and needle exchange programs. Disease in a baby can often be prevented by giving both the mother and child antiretroviral medication. Recognized worldwide in the early 1980s, HIV/AIDS has had a large impact on society, both as an illness and as a source of discrimination. The disease also has large economic impacts. There are many misconceptions about HIV/AIDS, such as the belief that it can be transmitted by casual non-sexual contact. The disease has become subject to many controversies involving religion, including the Catholic Church's position not to support condom use as prevention. It has attracted international medical and political attention as well as large-scale funding since it was identified in the 1980s. HIV made the jump from other primates to humans in west-central Africa in the early-to-mid-20th century. AIDS was first recognized by the U.S. Centers for Disease Control and Prevention (CDC) in 1981 and its cause—HIV infection—was identified in the early part of the decade. Between the first time AIDS was readily identified through 2024, the disease is estimated to have caused at least 42.3 million deaths worldwide. In 2023, 630,000 people died from HIV-related causes, an estimated 1.3 million people acquired HIV and about 39.9 million people worldwide living with HIV, 65% of whom are in the World Health Organization (WHO) African Region. HIV/AIDS is considered a pandemic—a disease outbreak which is present over a large area and is actively spreading. The United States' National Institutes of Health (NIH) and the Gates Foundation have pledged $200 million focused on developing a global cure for AIDS.

Alzheimer's disease (AD) is a neurodegenerative disease and is the most common form of dementia, accounting for around 60–70% of cases. The most common early symptom is difficulty in remembering recent events. As the disease advances, symptoms can include problems with language, disorientation (including easily getting lost), mood swings, loss of motivation, self-neglect, and behavioral issues. As a person's condition declines, they often withdraw from family and society. Gradually, bodily functions are lost, ultimately leading to death. Although the speed of progression can vary, the average life expectancy following diagnosis is three to twelve years. The causes of Alzheimer's disease remain poorly understood. There are many environmental and genetic risk factors associated with its development. The strongest genetic risk factor is from an allele of apolipoprotein E, a protein involved in the metabolism of fats in mammals. Other risk factors include a history of head injury, clinical depression, and high blood pressure. The progression of the disease is largely characterised by the accumulation of malformed protein deposits in the cerebral cortex, called amyloid plaques and neurofibrillary tangles. These misfolded protein aggregates interfere with normal cell function, and over time lead to irreversible degeneration of neurons and loss of synaptic connections in the brain. A probable diagnosis is based on the history of the illness and cognitive testing, with medical imaging and blood tests to rule out other possible causes. Initial symptoms are often mistaken for normal brain aging. Examination of brain tissue at postmortem used to be the only way to definitively diagnose Alzheimer's but neuroimaging techniques and fluid biomarkers have made diagnosis in vivo possible. No treatments can stop or reverse its progression, though some may temporarily improve symptoms. A healthy diet, physical activity, and social engagement are generally beneficial in aging, and may help in reducing the risk of cognitive decline and Alzheimer's. Affected people become increasingly reliant on others for assistance, often placing a burden on caregivers. The pressures can include social, psychological, physical, and economic elements. Exercise programs may be beneficial with respect to activities of daily living and can potentially improve outcomes. Behavioral problems or psychosis due to dementia are sometimes treated with antipsychotics, but this has an increased risk of early death. As of 2020, there were approximately 50 million people worldwide with Alzheimer's disease. It most often begins in people over 65 years of age, although up to 10% of cases are early-onset impacting those in their 30s to mid-60s. It affects about 6% of people 65 years and older, and women more often than men. The disease is named after German psychiatrist and pathologist Alois Alzheimer, who first described it in 1906. Alzheimer's financial burden on society is large, with an estimated global annual cost of US$1 trillion. Alzheimer's and related dementias are ranked as the seventh leading cause of death worldwide. Given the widespread impacts of Alzheimer's disease, both basic-science and health funders in many countries support Alzheimer's research at large scales. For example, the US National Institutes of Health program for Alzheimer's research, the National Plan to Address Alzheimer's Disease, has a budget of US$3.98 billion for fiscal year 2026. In the European Union, the 2020 Horizon Europe research programme awarded over €570 million for dementia-related projects.

Psychiatry is the medical specialty devoted to the diagnosis, treatment, and prevention of deleterious mental conditions. These include matters related to cognition, perceptions, mood, emotion, and behavior. Initial psychiatric assessment begins with taking a case history and conducting a mental status examination. Laboratory tests, physical examinations, and psychological assessments may also be used. On occasion, neuroimaging or neurophysiological studies are performed. Mental disorders are diagnosed in accordance with diagnostic manuals such as the International Classification of Diseases (ICD), edited by the World Health Organization (WHO), and the Diagnostic and Statistical Manual of Mental Disorders (DSM), published by the American Psychiatric Association (APA). The fifth edition of the DSM (DSM-5) was published in May 2013. However, its release coincided with a statement by then-NIMH director Thomas Insel noting that the manual lacks validity compared to other medical specialties due to the absence of objective laboratory measures (biomarkers) for diagnosis. Treatment may include psychotropics (psychiatric medicines), psychotherapy, substance-abuse treatment, and other modalities such as interventional approaches, assertive community treatment, community reinforcement, and supported employment. Treatment may be delivered on an inpatient or outpatient basis, depending on the severity of functional impairment or risk to the individual or community. Research within psychiatry is conducted by psychiatrists on an interdisciplinary basis with other professionals, including clinical psychologists, epidemiologists, nurses, social workers, and occupational therapists. Psychiatry has been controversial since its inception, facing criticism both internally and externally over its medicalization of mental distress, reliance on pharmaceuticals, use of coercion, influence from the pharmaceutical industry, and its historical role in social control and contentious treatments.

Diabetes mellitus, commonly known as diabetes, is a group of common endocrine diseases characterized by sustained high blood sugar levels. Diabetes tends to progress in severity, and is due to either a reduced production of the hormone insulin by the pancreas or unresponsiveness of bodily cells to insulin's effects. Classic symptoms include the three Ps: polydipsia (excessive thirst), polyuria (excessive urination), and polyphagia (excessive hunger), together with weight loss and blurred vision. If left untreated, the disease can lead to many health complications, including disorders of the cardiovascular system, eye, kidney, and nerves. The major types of diabetes are type 1 and type 2. The most common type is type 2. The most common treatment for type 1 is insulin replacement therapy (insulin injections), while anti-diabetic medications (such as metformin and semaglutide or tirzepatide) and lifestyle modifications can be used to manage type 2. Gestational diabetes, a form that sometimes arises during pregnancy, normally resolves shortly after delivery. Type 1 diabetes is an autoimmune condition in which the body's immune system attacks the beta cells (β-cell) in the pancreas, preventing the production of insulin. This condition typically is present from birth or develops early in life. Type 2 diabetes occurs when the body becomes resistant to insulin and thus, glucose remains in the bloodstream instead of being absorbed by the cells. Additionally, diabetes can result from other specific causes, such as genetic conditions (monogenic diabetes syndromes like neonatal diabetes and maturity-onset diabetes of the young), diseases affecting the pancreas (such as pancreatitis), or the use of certain medications and chemicals (such as glucocorticoids or other specific drugs, including those used after organ transplantation). The number of people worldwide diagnosed as living with diabetes has increased sharply in recent decades, from 200 million in 1990 to 828 million in 2024. Diabetes accounts for some two million deaths each year, and more than half of those affected are unaware of their condition. It affects one in seven of the adult human population, with type 2 diabetes accounting for more than 95% of cases. These numbers have already risen beyond earlier projections of 783 million adults by 2045. The prevalence of the disease continues to increase, most dramatically in low-middle income nations, where it is now the seventh leading cause of death. Rates are similar in women and men. The global expenditure on diabetes-related healthcare is an estimated US$760 billion a year.

Coronavirus disease 2019 (COVID-19) is a contagious disease caused by the coronavirus SARS-CoV-2. Starting in January 2020, the disease spread worldwide, resulting in the COVID-19 pandemic. In March 2020, the World Health Organization declared COVID-19 a global health emergency; they declared the end of the emergency in May 2023. The symptoms of COVID‑19 can vary but often include fever, fatigue, cough, breathing difficulties, loss of smell, and loss of taste. Symptoms may begin one to fourteen days after exposure to the virus. At least a third of people who are infected do not develop noticeable symptoms. Of those who develop symptoms noticeable enough to be classified as patients, most (81%) develop mild to moderate symptoms (up to mild pneumonia), while 14% develop severe symptoms (dyspnea, hypoxia, or more than 50% lung involvement on imaging), and 5% develop critical symptoms (respiratory failure, shock, or multiorgan dysfunction). Older people have a higher risk of developing severe symptoms and dying. Some people experience persistent symptoms (long COVID), for months or years after infection, including fatigue, cognitive issues and shortness of breath. Damage to organs has been observed in a subset. COVID‑19 transmission occurs when infectious particles are breathed in or come into contact with the eyes, nose, or mouth. The risk is highest when people are in close proximity, but small airborne particles containing the virus can remain suspended in the air and travel over longer distances, particularly indoors. Transmission can also occur when people touch their eyes, nose, or mouth after touching surfaces or objects that have been contaminated by the virus. People remain contagious for up to 20 days and can spread the virus even if they do not develop symptoms. There are two common tests to detect a COVID infection. Antigen tests (also called rapid lateral flow tests) can be used at home. A positive test indicates an active infection. However, negative test results are not always accurate, especially when there are no symptoms. Health care providers can perform a more accurate PCR test, which is typically analysed in a laboratory. Several COVID-19 vaccines have been approved and distributed in various countries, many of which have initiated mass vaccination campaigns. Other preventive measures include physical or social distancing, quarantining, ventilation of indoor spaces, use of face masks or coverings in public, covering coughs and sneezes, hand washing, and keeping unwashed hands away from the face. Initial treatment consists of drugs that have been developed to inhibit the virus for those at high risk and symptomatic treament, managing the disease through supportive care. The first known case was identified in Wuhan, China, in December 2019. Most scientists believe that the SARS-CoV-2 virus entered into human populations through natural zoonosis, similar to the SARS-CoV-1 and MERS-CoV outbreaks, and consistent with other pandemics in human history. Social and environmental factors including climate change, natural ecosystem destruction and wildlife trade increased the likelihood of such zoonotic spillover.

The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, such as viruses, bacteria, and parasites, as well as cancer cells and foreign objects, such as wood splinters—distinguishing them from the organism's own healthy tissue. Many species have two major subsystems of the immune system. The innate immune system provides a preconfigured response to broad groups of situations and stimuli. The adaptive immune system provides a tailored response to each stimulus by learning to recognize molecules it has previously encountered. Both use molecules and cells to perform their functions. Nearly all organisms have some kind of immune system. Bacteria have a rudimentary immune system in the form of enzymes that protect against viral infections. Other basic immune mechanisms evolved in ancient plants and animals (eukaryotes) and remain in their modern descendants. These mechanisms include phagocytosis, antimicrobial peptides called defensins, RNA interference, and the complement system. Jawed vertebrates, including humans, have a more sophisticated defense mechanism. For example, adaptive immunity provides the ability to adapt to recognize pathogens more efficiently (somatic hypermutation). This process creates an immunological memory, resulting in an enhanced response against subsequent encounters with that same pathogen. This mechanism of adaptive immunity is the fundamental basis of vaccination. Dysfunction of the immune system can cause autoimmune diseases, inflammatory diseases and cancer. Immunodeficiency occurs when the immune system is less active than normal, resulting in recurring and life-threatening infections. In humans, immunodeficiency can be the result of a genetic disease such as severe combined immunodeficiency, acquired conditions such as HIV/AIDS, or the use of immunosuppressive medication. Autoimmunity results from a hyperactive immune system attacking normal tissues as if they were foreign organisms. Common autoimmune diseases include Hashimoto's thyroiditis, rheumatoid arthritis, diabetes mellitus type 1, and systemic lupus erythematosus. Immunology covers the study of all aspects of the immune system.

A pandemic ( pan-DEM-ik) is an epidemic of an infectious disease that has a sudden increase in cases and spreads across a large region, for instance multiple continents or worldwide, affecting a substantial portion of the human population. Widespread endemic diseases with a stable number of infected individuals such as recurrences of seasonal influenza are generally excluded as they occur simultaneously in large regions of the globe rather than being spread worldwide. Throughout human history, there have been a number of pandemics of diseases such as smallpox. The Black Death, caused by the Plague, caused the deaths of up to half of the population of Europe in the 14th century. The term pandemic had not been used then, but was used for later epidemics, including the 1918 H1N1 influenza A pandemic—more commonly known as the Spanish flu—which is the deadliest pandemic in history. The most recent pandemics include the HIV/AIDS pandemic, the 2009 swine flu pandemic and the COVID-19 pandemic. Almost all these diseases still circulate among humans though their impact now is often far less. In response to the COVID-19 pandemic, 194 member states of the World Health Organization began negotiations on an International Treaty on Pandemic Prevention, Preparedness and Response, with a requirement to submit a draft of this treaty to the 77th World Health Assembly during its 2024 convention. The WHO Pandemic Agreement was adopted at the World Health Assembly in May 2025, establishing rules on how countries should act and collaborate to prevent, prepare for and respond to pandemic emergencies and public health emergencies of international concern. It was approved with 120 votes in favour, 70 abstentions and the absence of the United States, which had already withdrawn from the WHO.

Mental health encompasses emotional, psychological, and social well-being, influencing cognition, perception, and behavior. Mental health plays a crucial role in an individual's daily life when managing stress, engaging with others, and contributing to life overall. According to the World Health Organization (WHO), it is a "state of well-being in which the individual realizes their abilities, can cope with the normal stresses of life, can work productively and fruitfully, and can contribute to their community". It likewise determines how an individual handles stress, interpersonal relationships, and decision-making. Mental health includes subjective well-being, perceived self-efficacy, autonomy, competence, intergenerational dependence, and self-actualization of one's intellectual and emotional potential, among others. Mental health also includes emotional, psychological, and social well-being, and it affects how people manage stress and make everyday decisions. From the perspectives of positive psychology or holism, mental health is thus not merely the absence of mental illness. Rather, it is a broader state of well-being that includes an individual's ability to enjoy life and to create a balance between life activities and efforts to achieve psychological resilience. Cultural differences, personal philosophy, subjective assessments, and competing professional theories all affect how one defines "mental health".

Bacteria are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit the air, soil, water, acidic hot springs, radioactive waste, and the deep biosphere of Earth's crust. Bacteria play a vital role in many stages of the nutrient cycle by recycling nutrients and the fixation of nitrogen from the atmosphere. The nutrient cycle includes the decomposition of dead bodies; bacteria are responsible for the putrefaction stage in this process. In the biological communities surrounding hydrothermal vents and cold seeps, extremophile bacteria provide the nutrients needed to sustain life by converting dissolved compounds, such as hydrogen sulphide and methane, to energy. Bacteria also live in mutualistic, commensal and parasitic relationships with plants and animals. Most bacteria have not been characterised and there are many species that cannot be grown in the laboratory. The study of bacteria is known as bacteriology, a branch of microbiology. Like all animals, humans carry vast numbers (approximately 1013 to 1014) of bacteria. Most are in the gut, though there are many on the skin. Most of the bacteria in and on the body are harmless or rendered so by the protective effects of the immune system, and many are beneficial, particularly the ones in the gut. However, several species of bacteria are pathogenic and cause infectious diseases, including cholera, syphilis, anthrax, leprosy, tuberculosis, tetanus and bubonic plague. The most common fatal bacterial diseases are respiratory infections. Antibiotics are used to treat bacterial infections and are also used in farming, making antibiotic resistance a growing problem. Bacteria are important in sewage treatment and the breakdown of oil spills, the production of cheese and yogurt through fermentation, the recovery of gold, palladium, copper and other metals in the mining sector (biomining, bioleaching), as well as in biotechnology, and the manufacture of antibiotics and other chemicals. Once regarded as plants constituting the class Schizomycetes ("fission fungi"), bacteria are now classified as prokaryotes. Unlike cells of animals and other eukaryotes, bacterial cells contain circular chromosomes, do not contain a nucleus and rarely harbour membrane-bound organelles. Although the term bacteria traditionally included all prokaryotes, the scientific classification changed after the discovery in the 1990s that prokaryotes consist of two very different groups of organisms that evolved from an ancient common ancestor. These evolutionary domains are called Bacteria and Archaea. Unlike Archaea, bacteria contain ester-linked lipids in the cell membrane, possess elongation factors that are resistant to ADP-ribosylation by diphtheria toxin, use formylmethionine in protein synthesis initiation, and have numerous genetic differences, including a different 16S rRNA.

A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entity. Since Dmitri Ivanovsky's 1892 article describing a non-bacterial pathogen infecting tobacco plants and the discovery of the tobacco mosaic virus by Martinus Beijerinck in 1898, more than 16,000 of the millions of virus species have been described in detail. The study of viruses is known as virology, a subspeciality of microbiology. When infected, a host cell is often forced to rapidly produce thousands of copies of the original virus. When not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent viral particles, or virions, consisting of (i) genetic material, i.e., long molecules of DNA or RNA that encode the structure of the proteins by which the virus acts; (ii) a protein coat, the capsid, which surrounds and protects the genetic material; and in some cases (iii) an outside envelope of lipids. The shapes of these virus particles range from simple helical and icosahedral forms to more complex structures. Most virus species have virions too small to be seen with an optical microscope and are one-hundredth the size of most bacteria. The origins of viruses in the evolutionary history of life are still unclear. Some viruses may have evolved from plasmids, which are pieces of DNA that can move between cells. Other viruses may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity in a way analogous to sexual reproduction. Viruses are considered by some biologists to be a life form, because they carry genetic material, reproduce, and evolve through natural selection, although they lack some key characteristics, such as cell structure, that are generally considered necessary criteria for defining life. Because they possess some but not all such qualities, viruses have been described as "organisms at the edge of life" and as replicators. Viruses spread in many ways. One transmission pathway is through disease-bearing organisms known as vectors: for example, viruses are often transmitted from plant to plant by insects that feed on plant sap, such as aphids; and viruses in animals can be carried by blood-sucking insects. Many viruses spread in the air by coughing and sneezing, including influenza viruses, SARS-CoV-2, chickenpox, smallpox, and measles. Norovirus and rotavirus, common causes of viral gastroenteritis, are transmitted by the faecal–oral route, passed by hand-to-mouth contact or in food or water. The infectious dose of norovirus required to produce infection in humans is fewer than 100 particles. HIV is one of several viruses transmitted through sexual contact and by exposure to infected blood. The variety of host cells that a virus can infect is called its host range: this is narrow for viruses specialized to infect only a few species, or broad for viruses capable of infecting many. Viral infections in animals provoke an immune response that usually eliminates the infecting virus. Immune responses can also be produced by vaccines, which confer an artificially acquired immunity to the specific viral infection. Some viruses, including those that cause HIV/AIDS, HPV infection, and viral hepatitis, evade these immune responses and result in chronic infections. Several classes of antiviral drugs have been developed.