AIDS

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insidious, often fatal but less contagious disease than measles, chicken pox or hepatitis B. AIDS is thought to be caused primarily by a virus that invades white blood cells (lymphocytes) - especially T4-lymphocytes or T-helper cells - and certain other body cells, including the brain. In 1983 and 1984, French and U.S. researchers independently identified the virus believed to cause AIDS as an unusual type of slow-acting retrovirus now called "human immunodeficiency virus" or HIV. Like other viruses, HIV is basically a tiny package of genes. But being a retrovirus, it has the rare capacity to copy and insert its genes right into a human cell's own chromo- somes (DNA). Once inside a human host cell the retrovirus uses its own enzyme, reverse transcriptase, to copy its genetic code into a DNA molecule which is then incorporated into the host's DNA. The virus becomes an integral part of the person's body, and is subject to control mechanisms by which it can be switched "on" or "off". But the viral DNA may sit hidden and inactive within human cells for years, until some trigger stimulates it to replicate. Thus HIV may not produce illness until its genes are "turned on" five, ten, fifteen or perhaps more years after the initial infection.

During the latent period, HIV carriers who harbour the virus without any sign of illness can unknowingly infect others. On average, the dormant virus seems to be triggered into action three to six years after first invading human cells. When switched on, viral replication may speed along, producing new viruses that destroy fresh lymphocytes. As viral replication spreads, the lymphocyte destruction virtually sabotages the entire immune system. In essence, HIV viruses do not kill people, they merely render the immune system defenceless against other "opportunistic: infections, e.g. yeast invasions, toxoplasmosis, cytomegalovirus and Epstein Barr infections, massive herpes infections, special forms of pneu...