In a potential breakthrough that opens a new way to protect against HIV and other sexually transmitted diseases, researchers found that a gel applied by women before and after sex cut the chance of acquiring the AIDS virus by 39% and the genital herpes virus by 51%.

It is the first time an HIV-prevention method controlled by women, who bear the brunt of the epidemic in Africa, has been shown to work. About a third of women in the study said their partners didn't know they were using the clear, odorless gel.

The...