NEW YORK (AP) — A New York City woman infected her male partner with the Zika virus through sex, the first time female-to-male transmission of the germ has been documented.

Zika is usually spread by mosquitoes, and health officials have known for some time that men can spread it through sex. The Centers for Disease Control and Prevention reported the case Friday, and updated its advice for pregnant women.

The CDC now advises them to use protection if their sex partner has traveled to a Zika-infected region, whether the partner is a man or a woman.

The Zika virus causes only a mild illness, at worst, in most people. But infection during pregnancy can lead to severe brain-related birth defects for the fetus. The New York woman was not pregnant.

While this is the first documented case of a woman spreading Zika through sex, health experts say it is not surprising, because most diseases that can be spread through sex can be spread by both men and women.

It has likely been happening throughout the recent Zika outbreaks in Brazil, Latin America and elsewhere, though experts say it is probably not very common.

Last month, on the day the New York woman returned from a trip to a Zika-infected country, she had vaginal sex with her partner without a condom, health officials were told.

She went to her doctor three days after her return after developing common Zika symptoms such as fever, fatigue, rash and back pain. Tests showed Zika infection.

Seven days after they’d had sex, her male partner developed similar symptoms. He tested positive for Zika.