AIDS (acquired immune deficiency syndrome) is an infectious and fatal disease of apparently recent origin. AIDS is pandemic, which means that it is worldwide in distribution. A sufficient understanding of AIDS can be gained only by examining its causation (etiology), symptoms, treatments, and the risk factors for transmitting and contracting the disease.

AIDS occurs as a result of infection with the HIV (human immunodeficiency virus). HIV is a ribonucleic acid (RNA) virus that targets and kills special blood cells, known as helper T-lymphocytes, which are important in immune protection. Depletion of helper T-lymphocytes leaves the AIDS victim with a disabled immune system and at risk for infection by organisms that ordinarily pose no special hazard to the individual. Infection by these organisms is thus opportunistic and is frequently fatal.

The initial infection with HIV may entail no symptoms at all or relatively benign symptoms of short duration that...