TWO DECADES AGO, no one had ever heard of HIV or AIDS. Today HIV and AIDS are household words, known to practically every adult and schoolchild—in this country at least. But familiarity with AIDS has eliminated only some of the mystery and almost none of the controversy that accompanied the disease in its earliest days.

There was a time, not long ago, when no one knew for sure what caused AIDS or how it spread. So little was known about the disease that it didn't even have a name until 1982, when the English-speaking world chose "acquired immune deficiency syndrome" (later "acquired immunodeficiency syndrome"), or AIDS. Since that time, however, researchers have learned a great deal about AIDS. They know what causes it and how it spreads. They even know the genetic makeup of HIV, the virus that causes AIDS. And yet they still wrestle with the...