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Medical explanation of your assessments?

Dear Dr. Hunter:  Thank you for a great and very informative site.  I am male, and my questions relate to an explanation of the risk assessments you frequently give re: risk to a male in heterosexual acts with a woman.  

1)  I understand that you say a male performing oral sex on a woman (cunnilingus) is extremely tiny even if she did have HIV, like 1 in 10,000 or the like, and even that you are not aware of such a case ever having been documented, correct?  My question is – could you explain medically/biologically why it is so unlikely for a male to contract HIV through cunnilingus, what is it about the disease that makes this so highly unlikely?  If you don’t want to answer this, can you possibly point me to some site(s) that might explain why cunnilingus is so low-risk for the man?  Again, from the medical/biological perspective, what is it about the HIV virus that makes this so?

2)  Likewise, you say that for a man to catch HIV with unprotected vaginal intercourse with a woman one time, even if she were HIV positive, is like 1 in 1,000 (or 1 in 2,000?).  Again, medically, what is it about the disease that makes this so rare?  Also, just curious, where do these numbers come from?  Again, from the medical/biological perspective, what is it about the HIV virus that makes this so?  

3)  Finally, why is it that I am reading so much on the internet that makes it seem like these heterosexual acts are in fact very high-risk activities?  For example, I read one forum where a man asked about cunnilingus, and some nurse told him that the woman’s vaginal secretions were “highly infectious” and that he should go get tested.  I ask because it seems like most (or at least many) of us guys on this site are here due to freaking out about one or two incidents (“the condom came off with the CSW,” “unprotected sex with girl I just met” etc. etc.).

2 Responses
239123 tn?1267651214
As in many areas of human health, the epidemiologic data are more clear than the biological reasons behind them.  To some extent, my comments to follow are based on common sense and reasonable theory, not hard fact.  However, one principle is clear.  Many or most medically untrained persons assume that all it takes for an infection to take hold is exposure to "just one virus" (or bacterium).  Not true.  It takes exposure to lots of HIV for infection to occur, and those viral particles have to be introduced to the right kind of cells, typically directly into the blood or at least under the skin or mucous membranes.

To give you a specific example of what might seem a surprisingly low infection rate:  When a newborn without HIV is nursed by an infected mother, with the virus present in breast milk, on average only about 15% of babies become infected, and even that typically doesn't occur for 6 months.  That's with swallowing several ounces of HIV infected milk every day.

So the general answer to all three of your scenarios is that there simply isn't enough exposure of the virus to susceptible cells.

1) Cunnilingus:  The mouth and oral tissues are not susceptible to HIV, and as noted above, even swallowing HIV usually doesn't result in transmission.  When a woman has HIV, probably there isn't much virus in the cervical or vaginal secretions that make it out to the vulva.

2) All the risks you cite come from CDC and are based on pretty simple calculations:  how infected people say they were exposed, how often they performed those activities, and the statistical likelihood that their partners were infected. For vaginal sex, again, it has to do with the amount of virus in genital secretions and the exposure of HIV susceptible cells in the exposed person.  

3) There are two common sources of health information on the web.  Health departments and other government agencies typically err on the side of caution; they would rather warn people against even the lowest risk exposures than take the chance that someone is overly reassured and therefore gets infected.  The other main sources are those dominated by people with a political agenda or adverse personal experiences.  Nobody who performed oral sex 10,000 times goes on line to say "I blew 1000 guys 10 times each and still don't have HIV".  On the other hand, a single person who is convinced s/he acquired HIV by oral sex may repeat his or her story 100 times on 20 different websites.  Then of course there are plenty of well meaning sources that simply don't understand the biology of HIV or the epidemiologic data, and base their advice on what they believe to be common sense.

This is the sort of topic that can lead to lots of follow-up discussion, but this isn't the place for it.  But I hope this information helps put things in perspective.

Regards--  HHH, MD
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A related discussion, My kinda doc was started.
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