[C]ircumcision has been shown to reduce male-to-female HIV transmission by 60% to 75% .... A study ... of men living in South Africa finds that male circumcision might reduce the risk of men contracting HIV through sexual intercourse with women by about 60%.
So it works in both directions. Here is the Kaiser Foundation report the article quoted. The researchers for another study -- finding a 30% reduction in male-to-female transmission -- offered this graphic but memorable explanation and news of a further bonus:
[T]he reduced risk of transmission might be related to the structure of the foreskin, which can contain a concentration of the virus that is nine times the amount found in the outer layers of the penis .... Male circumcision also was found to reduce the rate of women's infection with trichomonas and bacterial vaginosis....
The Public Library of Science (PLoS) study that found a 61% reduction in female-to-male transmission concluded, "Male circumcision provides a degree of protection against acquiring HIV infection, equivalent to what a vaccine of high efficacy would have achieved." And what he doesn't get, ladies, he can't give to you. From the PLoS study's Introduction:
Male circumcision (MC) is associated with various cultural factors, including religious sacrifice, rites of passage into adulthood, and the promotion of hygiene. The earliest documentary evidence for circumcision is from Egypt. Tomb artwork from the Sixth Dynasty (2345–2181 B.C.) shows circumcised men, and one relief from this period shows the rite being performed on a standing adult male. Genesis (17:11) places the origin of the rite among the Jews in the age of Abraham, who lived around 2000 B.C.Well, the moral is plain in this cutting edge post.
Presently, MC practices in Africa are varied. Whereas men in Muslim countries are circumcised, as in North Africa or a large part of West Africa, in other societies the prevalence of MC depends on other cultural factors, such as changes that occurred under colonization.
UPDATE: Newsweek special report: How AIDS Changed America.
UPDATE: Circumcision may stop millions of HIV deaths-study.
UPDATE: Male circumcision 'lowers risk of HIV infection by 60%'.
UPDATE: Circumcision: Fact, Fiction and Hype.