A study has found that 50% of men are carrying a sexually-transmitted virus which has links to cancer.
The research found that half of men are infected with human papilloma virus (HPV) - triggering calls for boys to be vaccinated against it.
Experts have already established links between certain strains of HPV and cervical cancer in women as well as oral cancer.
The research, published in The Lancet, also found that around 6% of men every year contract the cancer-causing HPV 16 infection.
The study - conducted by scientists at the Cancer Centre and Research Institute in Florida - has prompted British experts to call for boys to be vaccinated to stem the spread of the virus and prevent genital warts and anal cancers, which are also thought to be caused by HPV.
The well-known link between HPV and cervical cancer has led to the routine vaccination of girls aged 12 to 13 in the UK since 2008.
Head of the College of Medical and Dental Sciences at the University of Birmingham, Professor Lawrence Young, said: "Although the study is a little unclear about how incidence is defined, it confirms something that we have suspected for some time - that men are a reservoir of HPV infection and transmit this virus to women."
The study analysed 1,159 men aged 18 to 70 years from the USA, Brazil, and Mexico. It found that men who had 50 or more female sexual partners were 2.4 times more likely to have the cancer-causing HPV infection compared to those with one partner or no partners.
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