U.S. health officials on Friday reported the first case of Zika spread through sex by a man with no symptoms of the disease.
In the other 21 U.S. cases of sexual transmission, the virus was spread by someone who at some point had symptoms.
The report details the case of a Maryland man who went to the Dominican Republic, where there is a Zika outbreak. He didn't get sick during the trip or when he returned. But his sex partner, who hadn't traveled, did get sick with Zika and recovered.
Zika is mainly spread by mosquitoes. Most infected people don't get sick. It can cause a mild illness, with fever, rash and joint pain. But infection during pregnancy can lead to severe brain-related birth defects.
Doctors believe spreading from an infected person with no symptoms is extremely rare; it may be that they have smaller amounts of virus in their blood and bodily fluids and are less infectious.
The Centers for Disease Control and Prevention advises couples to use condoms for two months after a partner without symptoms has returned from a Zika outbreak area, if the woman is of child-bearing age. For men with symptoms, the CDC advises condoms for at least six months.
The Maryland case is unusual and doesn't warrant a change in that advice, said the CDC's Dr. John T. Brooks.
French researchers in April reported a similar case of a couple without symptoms who both tested positive for Zika. But both had visited a Zika area so infection from mosquito bites couldn't be ruled out.