HOW DANGEROUS IS ORAL SEX?

Gay men tend to worry a lot more about the health hazards of anal sex than oral. Everybody knows, after all, that it's much easier for the virus that causes AIDS to cross the lining of the rectum than to infect someone through the mouth. Or is it? The surprising results of a study on rhesus monkeys published last week in Science not only suggest otherwise but also underscore how little scientists know about how, at the microscopic level, HIV spreads from one person to the next.

In the study, scientists from Boston's Dana-Farber Cancer Institute and Tulane University tried to...