Men infected with one type of genital human papillomavirus (HPV) may be at 20 times more at risk of reinfection with the same type, researchers say.HPV is the most common sexually transmitted infection and it is a major cause of genital warts and cancers of the genitals, mouth and throat.The findings showed that men who are infected once with HPV16 -- the type responsible for most HPV-related cancers -- are at 20 times higher risk of reinfection after one year, and 14 times higher after two years. The same effect was observed in both men who are sexually active and celibate, suggesting that they are not reacquiring the virus from another sexual partner.This increased risk suggests that infection confers no natural immunity against HPV, and emphasises the importance of vaccination for preventing the spread of HPV in young men before they become sexually active. Vaccination could potentially prevent reinfection in older men who have already contracted the virus, the researchers said."Vaccinating boys before HPV exposure could be a highly effective way to reduce the burden of HPV infection. Vaccinating men who have already been infected might also be effective," said lead author Sylvia Ranjeva, doctoral student at the University of Chicago.For the study, published in the Proceedings of the National Academy of Sciences, the team analysed data of over 4,000 unvaccinated men from three cities in Florida, Mexico and Brazil to understand what allowed over 200 genetically-distinct HPV types to coexist. The researchers found that the diversity of HPV types may stem from recurring infections of particular types within individuals. While relatively few people are infected with any one type, the high overall HPV prevalence occurs because nearly half the adult population carries at least one type of genital HPV. "The best thing we can do is prevent the initial infection by vaccinating boys before sexual contact. However, if the increased risk of reinfection is due to auto-inoculation, then another effective strategy may be to vaccinate previously infected men as well," Ranjeva said.