U.S. health officials are reporting the first case ofZikaspread through sex by a man with no symptoms of the disease.

In earlier cases of sexual transmission, the virus was spread by someone who at some point had symptoms.

Zika is mainly spread by mosquitoes, and causes only a mild illness in most people. But infection during pregnancy can lead to severe brain-related birth defects.

Friday's 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 but his sex partner did and recovered.

Doctors believe Zika spread from someone with no symptoms is extremely rare. They said the case is not reason enough to change health advice to couples.

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