Circumcision might reduce a man's risk of developing prostate cancer by 15 percent, according to new research published online March 12 in Cancer.

Of 1,754 men surveyed who had been diagnosed with prostate cancer, fewer—percentage-wise—had been circumcised than the 1,645 men who did not have prostate cancer. Men with more aggressive forms of prostate cancer were even more likely to be uncircumcised. Most men were 55 or older; to be protective against cancer risk, they had to have been circumcised before they first had sex.

The timing is important, the researchers suggest, because sexually transmitted infections might play a key role in making later cancer development more likely. Previous research showed that men who had any sexually transmitted infection were at a higher risk for prostate cancer.

Male circumcision has already been shown to reduce the risk of HIV infection as well as other sexually transmitted diseases. But the new correlation does not suggest that prostate cancer is necessarily caused directly by a viral infection, in the way penile and anal cancers have been linked to the human papillomavirus (HPV), which can be transmitted during various sexual activities. (Earlier research has pointed to a retrovirus, xenotropic murine leukemia virus-related virus, or XMRV, as a possible link for some aggressive cases.) Instead the infection can start a chain reaction that increases the odds of cancer. An underlying infection often leads to chronic inflammation in the body which in turn creates a more welcoming environment for cancer cells.

Circumcision nips this cycle in the bud in several ways. Foreskin is prone to small tears, which can allow viruses and bacteria to enter the body. This tissue also can harbor infectious strains that can get into the body later. Finally, after circumcision, the tissue in the circumcised area becomes tougher and less prone to damage and microscopic hangers-on.

The pattern suggests "a biologically plausible mechanism through which circumcision may decrease the risk of prostate cancer," Jonathan Wright of the University of Washington School of Medicine and co-author of the study said in a prepared statement. And the new data point to the need for further study. In the meantime, other risks have already been established, such as older age and family history. All these factors can help a doctor and patient best decide about screening via PSA (prostate-specific antigen) blood tests, a step the U.S. Preventive Services Task Force last year recommended against as standard procedure for healthy men.

Read more about prostate cancer in "The Great Prostate Debate" from the February 2012 issue of Scientific American.