Multiple studies have shown that circumcision decreases a man's risk of contracting HIV by as much as 60 percent, but there's been no concrete explanation as to why that is. New research, published yesterday in the journal mBio, supports the theory that this is due to decreased levels of bacteria. The paper shows that circumcision dramatically lowers the amount of bacteria in the area, which could be the reason for the increased resistance when compared to uncircumcised males.

The research team responsible from the study at the Translational Genomics Research Institute in Arizona looked at the levels of bacteria in over 156 uncircumcised men aged 15 to 49. Half of the volunteers were then circumcised, while the other half were not. After one year, there was an average 81-percent reduction in bacteria in the circumcised men. The team concludes that removing the foreskin positively changes the penile ecosystem by increasing the amount of oxygen that reaches the previously-covered skin. This in turn greatly decreases the prevalence of anaerobic bacteria (although slightly increases aerobic bacteria levels), reducing the risk of inflammation that can make HIV infection more likely. Although the team didn't directly look at HIV risks, the research suggests that the lower level of anaerobic bacteria "may play a role in HIV risk reduction."

The fact that circumcised people are less likely to contract HIV represents one of very few cases where modifying the body's natural form can have positive effects on health. Speaking with The Los Angeles Times, Lance Price, a co-author of the paper, explained that "as a society, we've gotten used to thinking about alterations to the microbiome as having negative outcomes," giving the example of antibiotics resulting in a gut infection. "But here's a situation where we're flipping that notion on its head. The disturbance of the microbiome could have a positive effect."