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The Ebola virus comprises a group of pathogenic agents that cause severe and deadly hemorrhagic fevers in humans and other primates. During 2014 and 2015, an Ebola virus epidemic in West Africa—predominantly, the countries of Guinea, Liberia, and…

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…

An age-contingent reduction in physiological function and activity, with a concomitant increase in mortality rate and a diminution in reproductive rate. Aging is the process of becoming older, which is influenced by genetic and environmental…

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 medical condition resulting from a reduction in the total quantity of hemoglobin or of red blood cells in the circulation. Anemia is a disorder marked by significant decreases in hemoglobin concentration and in the number of circulating red blood…

The application of genetic principles for improving heredity for economically important traits in domestic animals. The science of animal breeding is based on fundamental genetic concepts. Through controlled mating and selection, which can involve…

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 observation, measurement, and explanation of human variability in time and space. Anthropology is a science concerned with the interrelations of the biological, cultural, geographical, and historical aspects of humankind, including modern human…

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,…

The scientific discipline that studies the origin, evolution, distribution, and future of life in the universe. Astrobiology is an interdisciplinary science integrating contributions from biology, geology, astronomy, paleontology, and planetary…

An abundant type of glial cell of the central nervous system. Astrocytes (Fig. 1) are specialized glial cells (nonneuronal cells that surround neurons) of the central nervous system (CNS) that exert numerous essential functions in both health and…

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…

The occurrence in an organism of an immune response to one of its own tissues, that is, a response to a self constituent. Autoimmunity is an immune state in which antibodies are formed against the person's own body tissues. Foreign substances to…

The part of the nervous system that controls visceral (internal organ) functions of the body. The autonomic nervous system (see illustration) innervates smooth and cardiac muscles and the glands, and regulates visceral processes involuntarily,…

The class of animals consisting of the birds. Birds (Fig. 1) are vertebrate animals comprising the class Aves. (In some classification schemes, the term Neornithes may also be used when referring to modern birds.) Primarily adapted for flying, the…

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 study of gene structure and function in bacteria. Bacterial genetics is a scientific field concerned with the mechanisms of heredity in bacteria (Fig. 1). In general, genetics itself is concerned with determining the number, location, and…

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…

Any of the viruses that infect bacterial cells. Bacteriophages, often simply called phages, are discrete particles, with dimensions ranging from approximately 20 to 200 nanometers. A given bacterial virus can infect only one or a few related species…

Impairments or conditions in which binocular visual skills are dysfunctional and the eyes are unable to align properly. Humans, like all mammals, have true binocular vision. This means that, in health, the two eyes of an individual normally move in…

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,…

The production of ethanol from biomass via fungal enzymatic methods. Fungal enzymes are naturally occurring proteins that can cause certain chemical reactions to occur in plants, for instance in their structural and storage polysaccharides. Fungi,…

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…

The method of introducing microorganisms (native or genetically engineered) into a contaminated site to consume and break down environmental pollutants. Bioremediation is a form of biodegradation. Specifically, bioremediation is the use of…

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…

Any of the tubular channels for blood transport, of which there are three principal types: arteries, capillaries, and veins. Blood circulates through the body via blood vessels, which include the arteries, veins, and capillaries. These structures…

Disturbances in the structure and function of the skeletal system. Bone is the hard connective tissue that serves as the chief component of the vertebrate skeleton. In general, the musculoskeletal system provides a rigid structural frame that…

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.…

A crystalline methylxanthine alkaloid that acts as a stimulant to the central nervous system. Chemically, caffeine is 1,3,7-trimethylxanthine, and has the structural formula shown below. Caffeine is found in nuts, seeds, or leaves in over 60 plant…

A class of the phylum Porifera, including exclusively marine sponges with a skeleton composed of spicules of calcium carbonate to which can be added a solid basal calcitic skeleton. The sponges that constitute the phylum Porifera are assigned to…

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…