What are the odds of getting AIDS from ordinary heterosexual sex?

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Dear Cecil: Please provide some honest, realistic odds on a 100 percent heterosexual male contracting AIDS from a female by engaging in “traditional” (i.e., non-anal) intercourse — and I’m talking one-in-??? numbers. I am led to believe that a very low percentage of U.S. AIDS patients are women, and most of those are prostitutes or intravenous drug users. Assuming one exercises reasonable discretion in one’s choice of partners, the risk of infection has got to be slim. But I’m so confused by the subject I trust no one but you. Paranoid, Dallas


Illustration by Slug Signorino

Cecil replies:

The following numbers are old, and they’re all pretty much guesswork anyway, so take them with a grain of salt.  But I venture to say most people in the field would agree with my conclusions.

According to a report by researchers Norman Hearst and Stephen Hulley in the Journal of the American Medical Association, the odds of a heterosexual becoming infected with AIDS after one episode of penile-vaginal intercourse with someone in a non-high-risk group without a condom are one in 5 million. With a condom it’s even safer — one in 50 million. Just to put this in perspective, the chances of someone in your family getting injured next year in a bubble bath are 1 in 1.3 million (source: The Odds on Virtually Everything, Heron House, 1980). You’re in much greater danger of being struck by lightning (1 in 600,000), having your house bombed (1 in 290,000), or being murdered (1 in 11,000).

The numbers get a lot worse if you engage in “high-risk behavior” — having sexual intercourse or sharing needles with a member of a high-risk group, e.g., a gay or bisexual male or IV drug user from a major metro area, or a hemophiliac. The chances of getting AIDS from one such encounter range as high as 1 in 10,000 using a condom to 1 in 1,000 unprotected. Even if your partner tests negative for human immunodeficiency virus (HIV), the chances of infection from a high-risk person are still relatively high — 1 in 50,000 without a condom. That’s because there’s a 45 to 90 day window during which a newly infected person can infect others but test negative. (A few people go as long as 180 days.)

From there on out, statistically speaking, things deteriorate pretty fast. If your partner is HIV-positive, your chances of getting AIDS after one night are 1 in 5,000 with a condom, 1 in 500 without. Have sex with an HIV-positive partner 500 times using condoms and your chances escalate to 1 in 11. Skip the gift wrap and they’re 2 in 3.

A couple points: These odds apply equally to men and women. Although there’s reason to believe male-to-female AIDS transmission happens more often than female-to-male, the amount of difference is unknown. Also, I have to emphasize again, the numbers involve a lot of guesswork. The authors admit they could be off by a factor of ten in either direction. Still, one message comes through loud and clear: by far the best thing you can do to avoid AIDS is to be picky about your partners. Use of condoms reduces your risk by a factor of 10, sleeping only with people who test negative reduces it by a factor of 5 to 50, but avoiding high-risk partners reduces it by a factor of 5,000. (You’d also be well advised to avoid high-risk behavior, such as unprotected receptive anal sex.) Asking for a resume may not be romantic, but it beats Kaposi’s sarcoma.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.