WHEN THE FIRST cases of AIDS surfaced in the United States and Europe in the late 1970s and early 1980s doctors had no idea what they were dealing with. Nearly all the affected patients were previously healthy young men. The medical literature of the time made no mention of this group as likely candidates for the rare forms of pneumonia, skin cancer, and other conditions showing up in the doctors' offices. What the literature did say, however, was that these conditions—rare as they were—hardly ever killed anyone. And yet, one by one, these patients were dying horrible deaths.

The first cases were especially perplexing because they did not fit the pattern of any known disease. They exhibited no common set of symptoms and had no recognizable source. The only common characteristic seemed to be sexual orientation: nearly all...