NEW YORK — 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.

This microscope image from January provided by the Centers for Disease Control and Prevention shows the Zika virus.

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 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. Two days later, he went to the same doctor. The doctor tested him even though he hadn’t traveled from a Zika outbreak area and no cases of female-to-male transmission had been reported. He tested positive for Zika.

They both are in their 20s, but no other details about them were released, including where the woman traveled. Both have recovered, a CDC official said.

The woman began menstruating the day after they had sex. Health official say she may have spread the virus through vaginal fluid or menstrual blood.

The primary concern about Zika infection is the virus’s threat to pregnancies, and health officials have issued cautions to pregnant women who have a male sex partner who may have been infected.

The CDC has altered its advice to account for lesbian couples that include a pregnant woman. The CDC is now recommending that “barrier methods” be used by all pregnant women who have a sex partner who lives in or travels to a Zika outbreak area. No woman-to-woman sexual transmission had been reported to date.

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