Uncircumcised men who wash their penises directly after having unprotected sex may be at a heightened risk of contracting HIV, according to a new study.

The research, conducted in Uganda, studied more than 2,500 men and found that those who washed within three minutes of having sex had a 2.3 percent risk of HIV infection, while those who waited at least 10 minutes had a 0.4 percent risk.

Washing the penis after sex is generally encouraged in Africa as a part of good hygiene; some health experts have advocated the practice as a way of removing vaginal fluids that could potentially enter the body through an uncircumcised penis.

Researchers don’t know why the risk of infection increased after washing, but they say it might be because men are washing off acidic vaginal secretions that might prevent infection, or that the use of water may encourage viral survival and transmission.