As HIV expands its reach around the world, scientists are still trying to figure out basic information about the virus. For example, we all know that unsafe sex can transmit HIV— but the virus doesn't get transmitted 100 percent of the time. For each act of intercourse with an HIV-positive female partner, some studies have found that a man has less than a 1 percent chance of getting HIV; others have put that risk much higher. There is some evidence that being uncircumcised increases the risk of picking up HIV. Researchers working with a group of heterosexual Kenyan men looked at how circumcision affected the men's risk.

What the researchers wanted to know: How does being circumcised affect the risk of contracting HIV?

What they did: Between 1993 and 1997, 992 HIV-negative men who worked for six trucking companies in Mombasa, Kenya, joined a study on HIV. Every three months, the men came in for a visit. They answered questions about their sexual behavior since the last visit, including condom use and whom they'd had sex with. They were also examined for signs of sexually transmitted diseases and given a blood test for HIV. The men were followed up for a median of a year and nine months. Most of the men were married when the study started, but that didn't mean they had only one sexual partner—most of the married men reported having extramarital sex. The researchers checked when men went from being HIV-negative to HIV-positive and calculated the probability of contracting HIV from each act of intercourse.

What they found: Uncircumcised men were two to three times as likely to contract HIV from an HIV-positive woman as circumcised men (the risk varied a little depending on what assumptions the researchers made, for example about how many of the women were infected with HIV). They had about a 1.3 percent chance each time they had sex; circumcised men had only about a 0.5 percent chance. Since some people have suggested that the apparent difference in HIV transmission for circumcised and uncircumcised men really just shows a cultural difference between relatively clean-living Muslims (who are almost all circumcised) and non-Muslims, the researchers did the same calculations on only the men who weren't Muslim. They found roughly the same results. Other studies have suggested that the biology of the foreskin could be responsible for the increased risk; it's thin-skinned, making it vulnerable to the microabrasions that could occur during intercourse, and it has a large number of the cells that HIV initially targets.

What the study means to you: Studies like this help epidemiologists understand how HIV spreads—and could help suggest ways to control the virus. Many studies on transmission have been done on couples where one partner is HIV-positive and the other is HIV-negative, and they've come out with lower transmission rates; the researchers point out that if a couple has made it to the time the study starts without passing HIV from one to the other, maybe the HIV-negative partner has some resistance to the virus.

Caveats: The researchers could have missed some differences in sexual behavior or partners between circumcised and uncircumcised men. The researchers didn't know whether the men's sex partners were HIV-positive or not and had to make guesses based on national estimates of HIV prevalence. But they tried several different possible numbers and still came up with the same results. Also, note that these numbers are calculated per act of vaginal intercourse; a man might have only a 1.3 percent chance of contracting HIV each time he has sex with an HIV-positive woman, but if he has a lot of sex with many different women, his personal chance of getting HIV is far higher than 1.3 percent.

Find out more: Facts about HIV transmission from the U.S. Centers for Disease Control and Prevention

Read the article: Baeten, J.M., et al. "Female-to-Male Infectivity of HIV-1 Among Circumcised and Uncircumcised Kenyan Men." Journal of Infectious Diseases. Feb. 15, 2005, Vol. 191, pp. 546–553.

Article online: www.journals.uchicago.edu