You may have read that you can get the otherwise mosquito-borne Zika virus from having sex. Interestingly, all of the possible sexual-transmission cases in the U.S. involve men who traveled to Central or South America and had sex with their partners upon returning — there’s no evidence thus far that women can transmit the virus the same way.
Though the body is thought to clear the Zika virus in about a week, it seems to hang around in semen for much longer. This seems extremely unfair for people who have sex with men (and who didn’t get to go to the Caribbean), but as Vox explains, it might be an evolutionary mechanism.
Experts believe the testicles are immune privileged, meaning the body’s natural defense system gives them a pass from inflammation-causing attacks since they’re vital to reproduction. Immune cells from the blood can’t get in there and the intruder-fighting cells that do live inside aren’t as tough as the ones in the rest of the body, so a virus that somehow gets through is in a relatively safe space. Other immune-privileged areas of the body are the eyes, a fetus and placenta, and the central nervous system, including the brain — all are either important for personal survival or the survival of the species. (Researchers observed that Ebola also chilled out in the scrotum and could be transmitted sexually.)
This is why health officials are recommending that men who’ve traveled to Zika-affected areas and displayed symptoms of the virus (e.g., fever, rash, or joint pain) use condoms for all types of sex with partners who are pregnant or might be since Zika is linked to the birth defect microcephaly. It should be noted that doctors still believe that mosquito bites are the primary route of Zika infection.
At this point, experts don’t know exactly how long Zika can survive in semen, but U.K. researchers found it in a man’s semen 62 days after he first got sick. The U.S. doesn’t give instructions for how long to use condoms, but Britain’s National Health Service says six months for men who showed symptoms and 28 days for those who traveled to the region but didn’t get sick.