I've put together data from "Reducing the Risk of Sexual HIV Transimssion", Varghese et al, Sexually Transmitted Dieseases, January 2002 and several other sources I cited in other posts to come up with a more-or-less complete picture of the current state of the art in estimating HIV risks.

First we have the list of relative risks for various sex acts with insertive fellatio defined as 1.
  insertive fellatio = 1
  insertive vaginal sex = 10
  insertive anal sex = 13
  receptive fellatio = 2
  receptive vaginal sex = 20
  receptive anal sex = 100

To get infectivity (inf) multiply these numbers by 0.0001, i.e.,

  inf := relative_risk * 0.0001

The probability of becoming infected in a single act of sex is:

  p := r * f * inf

where:
  r is the infection rate in the population of potential partners
  f is the per-episode probability of condom failure
  inf is the infectivity of HIV for the specified act

The value of f is 1.0 if no condom is used. Estimates of f for HIV when a condom is used range from 0.03 to 0.15 depending on the source.

If the HIV relative risk of a BBBJ for the woman is 2 and her HIV relative risk of BBFS is 20 then her HIV relative risk of covered FS is 0.6 to 3.0 (depending on the value of f you use) and her HIV relative risk for a covered BJ is 0.06 to 0.3.

The bottom line is that a BBBJ is either somewhat more risky than covered FS for the woman or slightly less risky depending on how effective you believe condoms to be.

Finally, we have the list of values of r.
  r = 1.0 for a partner with a positive HIV test
  r = 0.1 for a male homosexual partner with unknown HIV status
  r = 0.01 for a hetrosexual partner with unknown HIV status
  r = 0.002 for a male homosexual partner with a negative HIV test
  r = 0.0002 for a hetrosexual partner with a negative HIV test

I would have expected the last two r values to be 0.0, but we live in an imperfect world and HIV tests are not perfect either.

You can plug the numbers into the formulas to get the risks for various combinations of factors. I have calculated the two extreme cases.

Worst case is doing receptive anal sex without a condom with a partner who has tested postive for HIV.

  p = 1.0 * 1.0 * (0.0001 * 100) = 0.01

Best case is doing insertive fellatio using a condom with a hetrosexual partner who has tested negative for HIV. (I will use 0.05 for the value of f as it seems to be the one in most common use. Personally, I don't think condoms are that effective and I tend to believe that an f of 0.15 is more realistic, but that is only my opinion.)

  p = 0.0002 * 0.05 * (0.0001 * 1) = 0.000000001

The bottom line is that the risks vary enormously depending on what you are doing, who you are doing it with, and whether or not you are using protection.

Note that this analysis is for HIV only.

Using the same relative risk scale and considering only vaginal intercourse, gonorrhea has a relative risk of about 2000 for men and 5000 for women. Syphillis has a relative risk of about 3000 and human papillomavirus has a relative risk of about 7000.

However, most STIs other than HIV are not life threatening and are much easier to treat. HIV is the 800 pound gorilla at the party.

HIV is kind of like a lottery where if you win you die. (Yes, I've read the Shirley Jackson story.) Under those circumstances you don't want a ticket no matter what the odds are against winning.

One of the other posters stated that statistics don't apply to individuals. I would state it a little differently, but it is important to remember that events of EXTREMELY low probability do happen to people all the time. The California Lottery is a good example. The odds against any particular ticket winning are millions to one. However, many weeks somebody wins. The odds against becoming HIV positive from any single sex act are very high but transmission happens and the disease continues to spread and people are dying -- especially in Africa.

There is no such thing as safe sex, but some sex is orders of magnitude safer than other sex. The key is in knowing what risks we are taking and keeping them to a level that we are comfortable with.

One obviously could avoid STIs by simply avoiding all sex. And some people believe that is exactly what we should try to teach our kids to do. However, there a lot of risks in life other than STIs and nobody lives forever. In the long run we are all dead. In the meantime there is a lot to be said for eating, drinking, and making merry. At least, that's my personal opinion. Others may differ.


-- Modified on 8/8/2002 1:36:12 PM