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 lead to the critical end point, acquired immune deficiency syndrome (AIDS). The AIDS disease is characterized by a profound progressive irreversible depletion of T-helper-inducer lymphocytes (CD4+ lymphocytes; Fig. 1), which leads to the onset of multiple and recurrent opportunistic infections by other viruses, fungi, bacteria, and protozoa, as well as various tumors (Kaposi's sarcoma and lymphomas). HIV infection is transmitted by sexual intercourse (vaginal, anal, or oral), by blood and blood products, and perinatally from an infected mother to a child (prepartum, intrapartum, and postpartum via breast milk). See also: Human immunodeficiency virus (HIV); Opportunistic infections

A type of defective virus particle in which part of the genetic material of papovavirus SV40 is encased within an adenovirus protein coat (capsid). Human adenoviruses require human cells for their propagation; however, papovavirus SV40 can serve as a “helper,” enabling human adenoviruses to replicate in monkey cells.

A family of double-stranded DNA viruses, collectively known as adenoviruses, which are icosahedrally symmetric, usually found in the respiratory tract of the host species, and often associated with respiratory diseases. Members of the Adenoviridae family (adenoviruses) have been associated with pharyngoconjunctival fever (types 3 and 7, in particular), acute respiratory disease (ARD) [types 4 and 7, in particular], epidemic keratoconjunctivitis (types 8, 19, and 37, in particular), and febrile pharyngitis in children (types 1, 2, and 5, in particular). A number of types have been isolated from tonsils and adenoids that were removed from surgical patients. Although most of the illnesses caused by adenoviruses are respiratory, adenoviruses are excreted frequently in stools, and certain adenoviruses have been isolated from sewage. Distinct mammalian serotypes (genus Mastadenovirus) and avian serotypes (genus Aviadenovirus) are known. See also: Animal virus; Eye disorders; Respiratory system disorders; Tonsillitis; Virus; Virus classification

A small infectious agent that is unable to replicate outside a living animal cell. Animal viruses infect animals and require animal cells as their host. Unlike other intracellular obligatory parasites (for example, chlamydiae and rickettsiae bacteria), animal viruses contain only one kind of nucleic acid, either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), but not both. They do not replicate by binary fission. Instead, they divert the host cell's metabolism into synthesizing viral building blocks, which then self-assemble into new virus particles that are released into the environment. During the process of this synthesis, viruses utilize cellular metabolic energy, many cellular enzymes, and cellular organelles that they are unable to produce. For this reason, they are incapable of sustaining an independent synthesis of their own components. The extracellular virus particle is called a virion (Fig. 1), whereas the name virus is reserved for various phases of the intracellular development. In addition, animal viruses are not susceptible to the action of antibiotics but, in some cases, may be susceptible to specific antiviral agents. See also: Antibiotic; Cell (biology); Deoxyribonucleic acid (DNA); Nucleic acid; Ribonucleic acid (RNA); Virus; Virus classification

Several brain-inflammation diseases, including Saint Louis encephalitis, Japanese encephalitis, and various forms of equine encephalitis, caused by arthropod-borne viruses (arboviruses). In their most severe human forms, the diseases collectively termed arboviral encephalitides (see table) invade the central nervous system and produce brain damage, with mental confusion, convulsions, and coma; death or serious aftereffects are frequent in severe cases. Inapparent infections are common. See also: Arthropoda; Disease ecology; Exotic viral diseases; Zoonoses

An infectious viral disease primarily affecting fowl, with rare occurrences in humans. Avian influenza, also termed bird flu, is a viral disease caused by influenza A subtypes that primarily affect chickens, turkeys, migratory waterfowl, and other avian species. Avian influenza outbreaks in domesticated birds are of concern for several reasons, including (1) the potential for rapid spread and significant illness and death among poultry during outbreaks, (2) the economic impact and trade restrictions resulting from an avian influenza outbreak, and (3) the possibility that avian influenza A viruses could be transmitted to humans. Two avian influenza viruses that have been of significant concern are the avian influenza subtype H5N1 virus, which is circulating primarily in parts of Asia and Africa, and the subtype H7N9 virus, which has been limited to China. See also: Animal virus; Aves; Infectious disease; Influenza; Poultry production; Public health; Virus; Zoonoses

Viruses that exclusively infect bacteria. Bacteriophages (Fig. 1), or phages, are the most abundant biological agents known, with an estimated population of 10³¹ on Earth. Phages are composed of either the nucleic acids DNA or RNA encapsulated in a protective protein capsid. Phages are highly selective in their bacterial targets and generally infect only a single species or even just a specific strain of a species of bacteria. As obligatory parasites, phages need a bacterial host for survival and reproduction. Since their discovery over a century ago, phages have played an instrumental role in both basic research and clinical applications. For example, the famous Hershey-Chase experiments—conducted in 1952 by U.S. biochemist Alfred Hershey and U.S. geneticist Martha Chase—used T2 phages to confirm that DNA, and not protein, serves as the heritable material of cells. More recently, as many pathogenic bacteria have developed resistance to antibiotics that are available for clinical use, researchers and clinicians are moving towards phage therapy, which uses phages to target pathogenic bacteria. See also: Bacteria; Virus

An arthropod-borne disease of ruminant species. The geographic distribution of bluetongue disease is dependent upon a susceptible ruminant population and climatic conditions that favor breeding of the primary vector, a biting midge (Culicoides species). See also: Arthropoda; Diptera; Disease ecology

Two different disease forms caused by the varicella-zoster virus. Chickenpox (also known as varicella) is a mild, highly infectious viral disease of humans caused by a herpesvirus and characterized by an itchy vesicular rash. Shingles (also termed zoster or herpes zoster) is a systemic virus infection affecting spinal nerve roots. It is characterized by vesicular eruptions distributed along the course of a cutaneous nerve and the skin surface that is supplied by the nerve. The varicella-zoster virus is a deoxyribonucleic acid (DNA) virus closely related to herpes simplex and Epstein-Barr viruses. Initial infection causes chickenpox, which is a common childhood infection characterized by fever, malaise, and a rash consisting of dozens to hundreds of small fluid-filled lesions (vesicles) that are individually surrounded by reddened skin (see illustration). Successive crops of lesions appear that eventually ulcerate and crust over during the approximately two-week course of the disease. The virus spreads from person to person by through the highly infectious respiratory secretions and lesion drainage. Chickenpox is rarely a serious disease in healthy children, butchildren but can be severe in immunocompromised individuals or in adults who escaped childhood infection. Primary infection results in immunity to a new varicella-zoster virus, but the original virus lies dormant in nerve ganglia cells. See also: Epstein-Barr virus; Herpes; Immunity; Immunology; Infectious disease; Nerve; Virus; Virus classification

A re-emerging arthropod-borne viral (arboviral) disease that is most often characterized by the sudden onset of high fever and severe joint pain (arthralgia), which can persist for years following acute illness. Chikungunya viruses are usually transmitted to humans by the bite of infected Aedes aegypti or A. albopictus mosquitoes (Fig. 1). Aedes species bite primarily during the daytime. Over the past decades, outbreaks of chikungunya virus disease (CVD) occurred in countries in Africa, Southeast Asia, and Europe, as well as on islands in the Indian Ocean and Pacific Ocean, resulting in millions of humans experiencing incapacitating severe joint pain (arthralgia). At the end of 2013, natural cases of CVD were confirmed in the Caribbean. The infections spread rapidly to other locations in the Americas within 9 months, resulting in a public health threat. See also: Animal virus; Arboviral encephalitides; Clinical pathology; Diptera; Disease ecology; Epidemiology; Infectious disease; Joint disorders; Mosquito; Public health; Virus; Virus classification; Zoonoses