Blood carries HIV from mouth to mouth.

A woman may have been infected with HIV from a man's bleeding gums during "deep kissing," reports the federal Centers for Disease Control and Prevention in Atlanta. The woman also had gum disease, which may have made it easier for the AIDS virus to enter her body. Scientists think that blood, not saliva, was the vehicle of transmission, emphasizes Scott D. Holmberg, a CDC epidemiologist.

The woman contracted the virus despite an apparent lack of behavior that would have put her at high risk, the CDC reports. The pair was part of a study of couples in which only one member was initially infected with HIV. Analysis of viral samples from the man and woman later in the study indicated that both were infected with the same strain, suggesting that HIV had passed from one to the other. Although no one can prove the route of transmission, the most likely possibility is that the virus in the man's bloody saliva infected the woman, says Nancy S. Padian, an epidemiologist at the University of California, San Francisco who directed the study.

This case shows that HIV may enter the body through the mouth, says Holmberg. "This has implications for couples who have unprotected oral sex and engage in deep kissing. We don't think transmission happens often this way, but it can."