SAN FRANCISCO (CBS SF) — Three new cases of the Zika virus among San Francisco residents have been diagnosed in the past two weeks, the city’s Department of Public Health announced Friday.

The number of San Francisco residents who have been diagnosed with the virus is now up to five. Each of the residents traveled in countries where Zika is circulating.

READ MORE: Candlelight Display At Golden Gate Park Honors Millions Of Lives Lost To AIDS

The virus, which was first reported as infecting a San Francisco resident on March 3 before another case was reported April 22, is not being contracted locally, as the mosquito-borne illness is not circulating here, health officials said.

“Since it is summer travel season, we want to remind San Franciscans who are planning to travel to countries where Zika virus is circulating to protect themselves from mosquito bites,” Dr. Tomas Aragon, San Francisco’s Health Officer, said in a statement.

Aragon added that pregnant women in particular should avoid “unnecessary travel” to the Latin American and Caribbean countries — and some of the Pacific Islands — where the virus has been prevalent.

Research into the exact risks posed by maternal Zika infection is ongoing, health officials said.

Meanwhile, a New York City woman has infected her male partner with 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.

READ MORE: COVID: Health Experts Analyzing Omicron Variant Before It Reaches U.S.

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. 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.

There is currently no vaccine available to prevent the infection, health officials said.

MORE NEWS: CA Drought: New Marin Water Restrictions Catch Some Residents Off Guard

TM and © Copyright 2016 CBS Radio Inc. and its relevant subsidiaries. CBS RADIO and EYE Logo TM and Copyright 2016 CBS Broadcasting Inc. Used under license. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. Bay City News Service contributed to this report.