Whence came the human immunodeficiency virus? We do not know, though there are several plausible theories -- as well as implausible conspiracy theories, that the virus was created in a US Government laboratory to attack gay men, or blacks, or both. The most plausible theories are that this is a mutant strain of a simian retrovirus which invaded humans in any of several possible ways; or that this is not a "new" virus at all, but one that has always been around, uncommon or rare as a cause of human disease and death. With modern population densities, migrations, changing social and sexual mores, it crossed the epidemic threshold, and the epidemic was launched. Reappraisal of preserved pathological specimens dating back to the late 1950s has yielded evidence suggestive of HIV infection, and a few clinical descriptions from earlier periods resemble AIDS.

HIV may turn out to be but one of many lethal micro organisms among the millions yet to be identified, taxonomically classified and systematically studied. Micro organisms outweigh humans by orders of magnitude in biomass alone. Some may not yet have encountered humans; others might mutate to harmful variants.