viroid \'vl-,r6id\ Infectious particle that is smaller than any of the known viruses. The particle consists of an extremely small circular RNA molecule that lacks the protein coat of a virus. Viroids appeal - to be transmitted mechanically from one cell to another through cellular debris. They are of much interest because of their subviral nature and their unknown mode of action. Viroids are agents of certain plant diseases; whether they occur in animal cells is uncertain.
virology \vl-'ra-l9-je\ Branch of microbiology that deals with the study of viruses. Viruses were not closely examined until 1892, when a Russian bacteriologist observed that the agent of tobacco mosaic disease could pass through a filter that did not permit the passage of bacteria. Modern virology began in the early 20th century, when the existence of bacterioph¬ ages was discovered. Direct visualization of viruses became possible after the electron microscope was introduced (c. 1940).
virtual reality Use of computer modeling and simulation to enable a person to interact with an artificial three-dimensional visual or other sen¬ sory environment. A computer-generated environment simulates reality by means of interactive devices that send and receive information and are worn as goggles, headsets, gloves, or body suits. The illusion of being in the created environment (telepresence) is accomplished by motion sen¬ sors that pick up the user’s movements and adjust his or her view accord¬ ingly, usually in real time. The basis of the technology emerged in the 1960s in simulators that taught how to fly planes, drive tanks, shoot artil¬ lery, and generally perform in combat. It came of commercial age in the 1980s and is now used in games, exhibits, and aerospace simulators. It has potential for use in many fields, including entertainment, medicine and biotechnology, engineering, design, and marketing.
virtue Practical dispositions in conformity with standards of excellence or with principles of practical reason. The seven cardinal virtues of the Christian tradition include the four “natural,” or cardinal, virtues, those inculcated in the old pagan world that spring from the common endowment of humanity, and the three “theological” virtues, those specifically pre¬ scribed in Christianity and arising as special gifts from God. The natural virtues are prudence, temperance, fortitude, and justice; this enumeration, said to go back to Socrates, is found in Plato and Aristotle. To these St. Paul added the theological virtues of faith, hope, and love—virtues which, in Christian teaching, do not originate naturally in humanity but are instead imparted by God through Christ and then practiced by the believer.
virtue ethics Approach to ethics that takes the notion of virtue (often conceived as excellence) as fundamental. Virtue ethics is primarily con-
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cemed with traits of character that are essential to human flourishing, not with the enumeration of duties. It falls somewhat outside the traditional dichotomy between deontological ethics and consequentiausm: It agrees with consequentialism that the criterion of an action’s being morally right or wrong lies in its relation to an end that has intrinsic value, but more closely resembles deontological ethics in its view that morally right actions are constitutive of the end itself and not mere instrumental means to the end. See also eudaemonism.
Virunga \ve-'rui]-ga\ Mountains Volcanic range, east-central Africa. Located north of Lake Kivu, it extends for about 50 mi (80 km) along the borders of Congo (Kinshasa), Rwanda, and Uganda. Of its eight major volcanic peaks, the highest is Karisimbi, while Nyiragongo is at the west¬ ern end of the chain; in 1861 John Hanning Speke was the first European to spot them. Parts of the range are in Virunga National Park, Volcanoes National Park (Rwanda), and Gorilla National Park (Uganda).
Virunga National Park formerly Albert National Park Game preserve and gorilla sanctuary, northeastern Congo (Kinshasa). Estab¬ lished in 1925, it has an area of about 3,050 sq mi (7,900 sq km). Its southern tip touches the northern shore of Lake Kivu, and much of its cen¬ tral region is occupied by Lake Edward. The Virunga Mountains lie between the two lakes. The park has been a UNESCO World Heritage Site since 1979.
virus Microscopic, simple infectious agent that can multiply only in liv¬ ing cells of animals, plants, or bacteria. Viruses are much smaller than bacteria and consist of a single- or double-stranded nucleic acid (DNA or RNA) surrounded by a protein shell called a capsid; some viruses also have an outer envelope composed of lipids and proteins. They vary in shape. The two main classes are RNA viruses (see retrovirus) and DNA viruses. Outside of a living cell, a virus is an inactive particle, but within an appropriate host cell it becomes active, capable of taking over the cell’s metabolic machinery for the production of new virus particles (virions). Some animal viruses produce latent infections, in which the virus persists in a quiet state, becoming periodically active in acute episodes, as in the case of the herpes simplex virus. An animal can respond to a viral infection in various ways, including fever, secretion of interferon, and attack by the immune system. Many human diseases, including influenza, the common cold, and AIDS, as well as many economically important plant and ani¬ mal diseases, are caused by viruses. Successful vaccines have been devel¬ oped to combat such viral diseases as measles, mumps, poliomyeutis, smallpox, and rubella. Drug therapy is generally not useful in controlling established viral infections, since drugs that inhibit viral development also inhibit the functions of the host cell. See also adenovirus; arbovirus; bacteriophage; picornavirus; plant virus; poxvirus. See illustration opposite.
virus, computer See computer virus
Visayan \vi-'sl-3n\ Islands Group of islands, central Philippines. Cov¬ ering 23,944 sq mi (62,015 sq km), they are surrounded by the Visayan, Samar, and Camotes seas. The main islands are Bohol, Cebu, Leyte, Mas- bate, Negros, Panay, and Samar. These islands and their smaller neigh¬ bours make up the central group of the Ph ilippine archipelago. Agriculture and fishing are important. The major urban centres are Cebu City on Cebu and Iloilo City on Panay.
Visby City (pop., 1994 est.: 21,462), southeastern Sweden. Located on the northwestern coast of the island of Gotland, it was the site of a Stone Age settlement c. 2000 bc. By the 12th century ad it was a member of the Hanseatic League, and in the 13th century it was one of the leading commercial centres of Europe; the city coined its own money and devel¬ oped an international maritime code. It declined after the Danish conquest of Gotland in 1361. A seaside resort, it has remains of medieval walls. Because of its remarkably well-preserved medieval ramparts and build¬ ings, Visby was designated a protected monument in 1810 and a UNESCO World Heritage site in 1995.
viscometer Wis-'ka-mo-toA Instrument for measuring the viscosity (resistance to internal flow) of a fluid. In one type, the time taken for a given volume of fluid to flow through an opening is recorded. In the capillary-tube viscometer, the pressure needed to force the fluid to flow at a specified rate through a narrow tube is measured. Other types depend on measurements of the time taken for a sphere to fall through the fluid, or of the force needed to rotate the inner cylinder of a pair (the space between the two cylinders being filled with the fluid under test), or of the rate at which oscillations of a disk vibrating in the fluid die out.
bacteriophage
" r otein spikes
head
base plate
RNA
DNA
influenza virus
envelope
The influenza virus possesses both a protein shell (capsid) and a lipid and protein envelope. The protein spikes of the envelope facilitate adherence and entry into the host cell. The capsid proteins determine the influenza virus type (A, B, C), and the highly variable proteins of the spikes and envelope determine the different strains within each type. The bacteriophage (bacterial virus) shown here has a head shaped like an icosahedron (with 20 sides). The tail fibres attach the virus to the bacterium, bringing the base plate into contact with the surface. The tail contracts, and DNA from the head is injected into the host.