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Circumcised men are less likely to transmit and acquire HIV--as long as they continue to practice safe sex.

Mass circumcision campaigns in sub-Saharan Africa have attempted to stem the tide of HIV infection by making men less infectious. Unsafe behaviors after circumcision, however, offset the benefits of circumcision.


In the United States, a majority of males are still circumcised (the foreskins of the penises surgically removed) at birth. Outside of the Muslim and Jewish worlds, however, male circumcision has always been a relative rarity, until scientists in the last decade learned that men who have been circumcised are less likely to spread HIV.

Circumcision Changes the Kinds of Bacteria That Live on the Penis

Probably the strongest predictor of whether a sexually active adult will be infected with the virus that causes AIDS is whether or not he or she already has been infected with the herpes virus. In studies of HIV transmission in India, researchers have found that men who are already infected with herpes are 2.5 to 14 times more likely to become infected with HIV when they are exposed to it, and women who are already infected with herpes are 1.4 to 2.8 times more likely to become infected with HIV when they are exposed the virus. But there are also factors that reduce the likelihood of exposure.

One of those factors is the presence of certain kinds of bacteria on the male penis. The penis is always inhabited by surface bacteria, but different kinds of bacteria predominate when the surface of the penis is exposed to the air, and when it is not. When a man has not been circumcised, most of the bacteria at the tip of his penis (the region known in science as the “coronal sulcus”) are anaerobic, that is, bacteria that do not depend on the air as their source of oxygen. When a man has been circumcised, most of the bacteria at the tip of his penis are aerobic, or oxygen-loving.

The Immune System Responds Differently in Circumcised and Uncircumcised Men

The immune system responds to anaerobic and aerobic bacteria in different ways. Generally, aerobic, air-loving bacteria are less threatening to the immune system, so the white blood cells in the skin of the penis are not as highly activated as they are in the presence of anaerobic bacteria. When white blood cells are not as active, they are less likely to bind to HIV, and there is less surface area on the penis that in which the immune system can become activated.  Consequently a man who has been circumcised is less likely to be infected by HIV and then less likely to spread HIV to his sex partners. Men who are circumcised are far less likely to acquire HIV infections during unprotected sex.

Circumcision does not appreciably change the risk of becoming infected by or infecting one's partner with gonorrhea, syphilis, or the Trichomonas microbe that causes severe urinary tract infections. Among sexually transmitted diseases, it just reduces the risk of HIV, although it reduces the risk of HIV transmission by 50% to 60%. But in countries where mass circumcision campaigns have been attempted to reduce the risk of spreading HIV, HIV rates have not necessarily gone down. The reason has to do with behavior.

Continue reading after recommendations

  • Arora P, Nagelkerke NJ, Jha P. A systematic review and meta-analysis of risk factors for sexual transmission of HIV in India. PLoS One. 2012. 7(8):e44094. doi: 10.1371/journal.pone.0044094. Epub 2012 Aug 28.
  • Liu CM, Hungate BA, Tobian AA, Serwadda D, Ravel J, Lester R, Kigozi G, Aziz M, Galiwango RM, Nalugoda F, Contente-Cuomo TL, Wawer MJ, Keim P, Gray RH, Price LB. Male circumcision significantly reduces prevalence and load of genital anaerobic bacteria. MBio. 2013 Apr 16. 4(2). pii: e00076-13. doi: 10.1128/mBio.00076-13.
  • Photo courtesy of Martin Lopatka by Flickr : www.flickr.com/photos/apothecary/4996039811/
  • Photo courtesy of Paul Keller by Flickr : www.flickr.com/photos/paulk/1241749044/