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A viral disease of humans caused by the human immunodeficiency virus (HIV), which attacks and compromises the body's immune system. Individuals infected with the human immunodeficiency virus (HIV) proceed through a spectrum of stages that ultimately…

Any of the organic compounds possessing one or more basic amino groups and one or more acidic carboxyl groups. Of the more than 80 amino acids that have been found in living organisms, approximately 20 serve as the building blocks for proteins.…

Synthetic manufactured drugs derived from the male sex hormone testosterone. Androgenic-anabolic steroids, often termed simply as anabolic steroids, are artificially produced testosterone variants. They can provide a number of medical benefits when…

A method used by animals to conceal themselves from an enemy by appearing to be a section of the natural background. Animal camouflage is a strategy that animal organisms use to avoid or deflect the attacks of predators by misleading the latter's…

An acute, infectious worldwide zoonotic disease (transmissible to humans) caused by the spore-forming bacterium Bacillus anthracis to which most animals, especially grazing herbivores, are susceptible. Anthrax is a serious, though rare, infectious…

An ongoing current event in which a large number of living species are threatened with extinction or are going extinct because of environmentally destructive human activities. The Earth is presently in the midst of a mass extinction event. In the…

The capability that a bacterium acquires to counteract an inhibitory chemical molecule or compound that was formerly effective in killing it or preventing its growth. Antibiotic resistance in bacteria (Fig. 1) is a hot topic in the media and in…

A chemical substance used to destroy or prevent the growth of infectious microorganisms on or in a human or other animal body. An antiseptic is an antimicrobial compound or drug preparation that inhibits the spread and development of harmful…

Programmed cell death triggered by extracellular signals or genetically determined events and carried out by physiological processes within the cell. Apoptosis is one of the major forms of cell death in multicellular organisms (necrosis is the other…

A group of diverse microscopic prokaryotic organisms that constitute one of the three domains of life. Taxonomists recognize three domains of cellular life: Archaea, Bacteria, and Eukaryota (also termed Eukarya or Eucarya) [Fig. 1]. The members of…

An allergic inflammatory disease of the pulmonary airways, marked by labored breathing, wheezing, and coughing. Asthma is a serious pulmonary disease that inflames and narrows the airways of the lungs (Fig. 1). The inflammation involves mast cells,…

A neurodevelopmental condition that impairs the way that a person relates to and communicates with other people. Persons with autism have difficulties with social communication and interaction. They also can have unusual behaviors, such as…

Extremely small, relatively simple microorganisms possessing the prokaryotic type of cell construction. Bacteria are very small prokaryotic microorganisms, usually measuring 0.3 to 2.0 micrometers in diameter (Fig. 1). They are found almost…

The biochemical reactions that together enable bacteria to live, grow, and reproduce. Strictly speaking, bacterial metabolism describes the total chemical reactions that take place in a bacterial cell, whereas bacterial physiology describes the role…

The classification, nomenclature, and identification of bacteria. Bacterial taxonomy is concerned with the naming of bacterial organisms and with organizing these names according to various criteria. Biologically, bacteria (Fig. 1) are prokaryotic…

A method for quantitatively determining the concentration, biological activity, or potency of a substance by its effect on the growth of a suitable animal, plant, or microorganism under controlled conditions. A bioassay (biological assay) is a…

The biological diversity of different plant and animal species living on the island of Borneo. Borneo is the third largest island in the world. Located in Southeast Asia and straddling the Equator, it is divided among three countries: Indonesia,…

A discipline concerned with the application of ethics to biological problems, especially in the field of medicine. Bioethics is a broad term that encompasses the environmental, ethical, legal, and social implications of agriculture, life sciences,…

A microbial (often bacterial) community that is enveloped by an adhesive substance (the glycocalyx) at the interface of a liquid and a surface. Biofilms (see illustration) are defined as highly structured communities of microorganisms that are…

Any of the self-sustained circadian (approximately 24-hour) rhythms that regulate daily activities, including sleep and wakefulness. A biological clock is any physiologic factor that functions in regulating innate organismal rhythms. Biological…

A biochemical reaction involving the transfer of a negatively charged electron from one organic compound to another organic compound or to oxygen. Biological oxidation is an energy-producing reaction in living cells, and it is coupled with a…

A natural science concerned with the study of all living organisms. Living organisms, from the viewpoint of biology (biological science), share a number of unifying themes, including their origin from the same basic cellular structure and their…

Generally, any technique that is used to make or modify the products of living organisms in order to improve plants or animals or to develop useful microorganisms. According to the general definition, biotechnology has actually been practiced for…

A major mental disorder in which there are lifelong episodes of both mania and depression; also known as manic-depressive illness. Bipolar disorder, also termed manic-depressive illness or manic depression, is characterized by sudden and often…

Any of the genetically determined markers on the surface of cellular blood elements (red and white blood cells, and platelets). Blood groups (types) have medical, legal, and anthropologic importance. In medicine, the matching of ABO and Rh groups of…

A collection of specialized cells (neurons) in the head that regulates behavior as well as sensory and motor functions. The brain is the enlarged anterior portion of the central nervous system in most bilaterally symmetrical animals, controlling and…

Malignant and benign lesions of the human mammary glands. Breast cancer and other breast disorders are potentially serious medical conditions. Benign breast disorders are often symptomatic and bothersome, but do not have malignant potential.…

The common name for a malignant neoplasm or tumor. Cancer is a serious medical condition that develops when the orderly relationship of cell division and cell differentiation becomes disordered, leading to the development of neoplasms. Neoplasms are…

The sum of the physical and chemical processes by which cancerous cells are produced and maintained, and by which energy is made available for their use. A cancer cell and a normal cell differ in several ways. One important and distinguishing…

Cells within a tumor that possess the capacity to self-renew and cause the heterogeneous lineages of cancerous cells that comprise the tumor. Cancer stem cells (CSCs) typically represent a small percentage of all cells in a tumor (rarely more than…

The sum of the biochemical and physiological processes involved in the breakdown and synthesis of simple sugars, oligosaccharides, and polysaccharides, and in the transport of sugars across cell membranes and tissues. The basic chemistry of…

The science of the electrical activity of the heart. Cardiac electrophysiology is the branch of cardiology concerned with the mechanism, spread, and interpretation of the electric currents that arise within heart muscle tissue and initiate each…

Common pigments, typically of yellow, orange, or red hues, that serve in both light absorption and the protection against too much light in photosynthesis as well as in the vision process. Carotenoids comprise a class of labile, easily oxidizable,…

The microscopic functional and structural unit of all living organisms. Cells can be separated into prokaryotic and eukaryotic categories. Eukaryotic cells contain a nucleus. They comprise protists (single-celled organisms), fungi, plants, and…

The cultivation of cells in the laboratory. Cell culture is the growth of living cells or microorganisms in a controlled artificial environment. Bacteria and yeasts may be grown suspended in a liquid medium or as colonies on a solid medium (usually…

Any of the thin semipermeable layers of protoplasm, consisting mainly of lipids and proteins, which are present on the surface of all cells. Cells maintain their content separate and distinct from the external environment by a semifluid lipid…

Charles Robert Darwin (February 12, 1809 – April 19, 1882) is one of the most celebrated and eminent scientists of the past few centuries, with his broadest and most notable influence arising from his theory of evolution by means of natural…

A severe diarrheal disease caused by infection of the small bowel of humans with Vibrio cholerae. Cholera is an acute, infectious bacterial disease that results in serious, often fatal, gastrointestinal illness in humans. The causative agent is…

Any persistent pain occurring in the lumbar (lower) back, sometimes extending to the buttocks and legs, which has been present for at least 3 months. Chronic low back pain (CLBP) [Fig. 1] is usually a combination of musculoskeletal and neuropathic…