Many viruses are known to cause worse illness in men than women. For example, human papilloma virus infections are 5 times more likely to cause cancer in men than in women, and Epstein-Barr virus infections are twice as likely to cause Hodgkin's lymphoma in men than in women. Although many people credit such divergences to sex differences in the immune system, a new proposed explanation owes them to the ability of women to transmit viruses more.
Researchers Francisco Ubeda and Vincent Jansen at Royal Holloway University of London theorize that viruses may cause less severe disease in women than in men to increase transmission. Women may be considered more valuable hosts since they have the potential to transmit viruses during pregnancy, birth, and breast-feeding. Using mathematical modeling, the researchers have shown that natural selection should favor weaker illness in women as long as they can transmit the virus to their offspring. If viruses are indeed adapting to sex and behaving differently in women and men, there is an even greater need to conduct clinical trials on both men and women (as opposed to just testing on male cohorts).