In terms of HIV this is very low risk. Theoretically transmission is possible but there has never been a well-documented case of transmission. Studies which have followed up couples in which one partner has HIV and the other does not have not identified any cases of HIV transmission due to cunnilingus (the vagina being licked).
The theoretical potential stems from the possible presence of HIV in vaginal fluids and menstrual blood. However the mouth and esophagus are quite thick and contain relatively few cells that would be vulnerable to HIV infection. Saliva may contain an enzyme that inhibits HIV infection.
In terms of an HIV positive person licking a woman's vagina and posing a risk to the woman, this is pretty implausible. There have never been any reported cases, even poorly documented ones.
But it's worth remembering that other sexually transmitted infections CAN be passed on through oral sex. Herpes, syphilis and gonorrhea are the STIs most commonly transmitted this way.
More on Oral Sex at TheBody.com
To find out more about the risk from oral sex, we recommend the following articles:
In addition, our Q&A experts sometimes address questions about oral sex in our "Ask the Experts" forums. Here are some of those questions and our experts' responses:
- Licking of the vagina
Is it possible for me to get HIV from my vagina being licked by an HIV positive man? Is the risk greater if he has brushed his teeth?
- Clit-licking and possible exposure to vaginal fluids
I have had a sex recently with my friend and I was licking her clit (only on top, didn't lick her vagina inside) and 30 minutes before that I smoked a cigarillo (may this have cause micro cuts in my mouth and therefore increase the risk)?