V3 LOOP: Section of the gp120 protein on the surface of HIV. Appears to be important in stimulating neutralizing antibodies. See also Antibodies; gp120.

VACCINATION: Inoculation of a substance (vaccine) into the body for the purpose of producing active immunity against a disease. The vaccine is usually a weakened culture of the agent causing the disease; the use of vaccines is a cornerstone of preventive medicine. Vaccination was used in ancient times in China, India and Persia, and was introduced to the West in the late 18th century by E. Jenner. Vaccinations have eradicated smallpox and are used today to prevent diphtheria, poliomyelitis, rabies and typhoid. Experimental vaccines for certain cancers have been developed for laboratory mice. See also Inoculation.

VACCINE: A substance that contains antigenic components from an infectious organism. By stimulating an immune response (but not disease), it protects against subsequent infection by that organism. See also Antigen; Vaccination.

VACCINIA: A cowpox virus, formerly used in human smallpox vaccines. Employed as a vector in HIV vaccine research to transport HIV genes into the body. See also Vaccination; Vector.

VECTOR: A nonpathogenic bacterium or virus used to transport an antigen into the body to stimulate protective immunity (e.g., in a vaccine). See also Antigen.

VIRAL BURDEN: (Viral Load). The amount of HIV virus in the circulating blood. Monitoring a person's viral burden is important because of the apparent correlation between the amount of virus in the blood and the severity of the disease: sicker patients generally have more virus than those with less advanced disease. A new, sensitive, rapid test-called the branched DNA assay for HIV-1 infection-can be used to monitor the HIV viral burden. In the future, this procedure may help clinicians to decide when to give anti-HIV therapy. It may also help investigators determine more quickly if experimental HIV therapies are effective.

VIRAL CORE: 1. Typically a virus contains an RNA (ribonucleic acid) or DNA (deoxyribonucleic acid) core of genetic material surrounded by a protein coat. See also DNA; Ribonucleic Acid. 2. As related to HIV: Within HIV's envelope is a bullet-shaped core made of another protein, p24, that surrounds the viral RNA. Each strand of HIV RNA contains the virus' nine genes. Three of these-gag, pol and env-are structural genes that contain information needed to make structural proteins. The env gene, for example, codes for gp160, a protein that is later broken down to gp120 and gp41. See also env; gp41; gp120; gp160; p24.

VIRAL CULTURE: A laboratory method for growing viruses.

VIRAL ENVELOPE: As related to HIV: HIV is spherical in shape with a diameter of 1/10,000 of a millimeter. The outer coat, or envelope, is composed of two layers of fat-like molecules called lipids, taken from the membranes of human cells. Embedded in the envelope are numerous cellular proteins, as well as mushroom-shaped HIV proteins that protrude from the surface. Each mushroom is thought toconsist of a cap made of four glycoprotein molecules called gp120, and a stem consisting of four gp41 molecules embedded in the envelope. The virus uses these proteins to attach to and infect cells.

VIRAL LOAD: See Viral Burden.

VIREMIA: The presence of virus in the bloodstream.

VIRION: A virus particle existing freely outside a host cell. A mature virus.

VIROLOGY: The study of viruses and viral disease.

VIRUCIDE: Any agent that destroys or inactivates a virus.

VIRUS: Organism composed mainly of nucleic acid within a protein coat, ranging in size from 100 to 2000 angstroms (unit of length; 1 angstrom is equal to 10-10 meters); they can be seen only with an electron microscope. During the stage of their life cycle when they are free and infectious, viruses do not carry out the usual functions of living cells, such as respiration and growth; however, when they enter a living plant, animal or bacterial cell, they make use of the host cell's chemical energy and protein- and nucleic acid-synthesizing ability to replicate themselves. Viral nucleic acids are single- or double-stranded and may be DNA (deoxyribonucleic acid) or RNA (ribonucleic acid). After viral components are made by the infected host cell, virus particles are released; the host cell is often dissolved. Some viruses do not kill cells but transform them into a cancerous state; some cause illness and then seem to disappear, while remaining latent and later causing another, sometimes much more severe, form of disease. Viruses, known to cause cancer in animals, are suspected of causing cancer in humans. Viruses also cause measles, mumps, yellow fever, poliomyelitis, influenza and the common cold. Some viral infections can be treated with drugs. See also DNA; Nucleic Acid; Ribonucleic Acid.