It is now well established that many viruses wreak more havoc on men than on women. Examples: Men are five times as likely to develop cancer from HPV as women, twice as likely to develop Hodgkin's lymphoma from the Epstein-Barr virus, and 1.5 times as likely to die of tuberculosis, per New Scientist. Scientists puzzling over why have suggested that women could have stronger immune systems, which would make sense from an evolutionary perspective given the importance of being able to bring offspring to term and nurse them through infancy. But now researchers using mathematical modeling say it might be the viruses themselves that have evolved to behave differently in their hosts depending on sex, and that includes in other animals like chickens, they report in the journal Nature Communications.
Viruses spread between hosts by making more copies of themselves. But in doing so, they also make their hosts ill, which doesn't actually behoove them. "That's not something a pathogen particularly sets out to do because it's shooting itself in the foot," says one researcher. Because a virus so easily jumps from mother to fetus or infant, women are the superior host, and thus there is evolutionary pressure to harm them less, Reason reports. How a virus can determine the sex of its host is a mystery, though subtle differences in hormones and other pathways could play a role. If researchers can sort this out, they might be able to trick viruses into thinking they're always infecting women, and get them to go easy on the men, too. (These are the deadliest viruses to infect humans.)