Not all semen infected alike.

Scientists know that the AIDS-causing virus (HIV) spreads most commonly through sexual contact. But many researchers have found this route mysteriously inconsistent: Some HIV-infected men spread the virus after having sex only once, while others fail repeatedly to pass the infection on to their partners.

In an effort to untangle these conflicting results, Deborah J. Anderson of Brigham and Women's Hospital in Boston and her co-workers collected semen samples from 95 HIV-infected men. Only nine of the samples contained the virus, the researchers report in the May 27 JOURNAL OF THE AMERICAN MEDICAL ASSOCIATION.

Then the group asked: Are some men more likely than others to have HIV in their semen? The researchers found that men with AIDS were more likely to have HIV in their semen than infected men with no symptoms of the disease. Moreover, men undergoing zidovudine therapy were more likely to test negative for semen-borne HIV than men who weren't using the drug. This suggests that zidovudine may provide some protection against sexual transmission of the virus, Anderson says.

In a related experiment, the group tracked 14 HIV-infected men over eight months, testing each man's semen for HIV once each month. Surprisingly, the team found that some men's semen tested positive for HIV some months but not others. Although she doesn't know why this happens, Anderson says it shows that a single negative semen sample is not always reliable.

How does HIV get into semen in the first place? Anderson's team found that many men whose semen harbored HIV also had a genital tract inflammation -- and resulting white blood cells in their semen. Since scientists know that HIV can stow away inside white blood cells, Anderson's group speculates that these cells may carry the virus into the semen.

Regardless of whether a man's semen contains the virus or not, cautions Anderson, "all men with HIV have the potential to be infectious."