Researchers studying the Zika virus have more bad news for anyone who has been exposed to the disease: in 14 cases in the United States, the Center for Disease Control has proved the virus was sexually transmitted from a male patient to a female patient, sometimes before the man has developed symptoms.

When a man is infected with Zika, the virus takes hold in his blood and other bodily fluids, including semen. The virus can lie dormant in semen for longer than it exists in blood but the CDC isn’t sure how much longer it can linger, or whether men who never develop symptoms can pass on the virus.

In each of the 14 confirmed cases, the virus was passed from an infected man to a female sexual partner during unprotected sex. The CDC also says it isn’t sure if infected women can pass the disease to their sexual partners. The virus isn’t particularly deadly to adults, and only produces symptoms in about 20 percent of patients. The CDC also isn’t sure if asymptomatic men can pass the disease — if they can, it would make controlling the disease much more complicated, as it could spread from infected men to their sexual partners without the men knowing they were infected.

Brazilian soldiers pass out informational leaflets to beachgoers.

At first, sexual transmission of the virus seemed rare, even when isolated cases occurred in the United States. But now, the number of confirmed cases has risen to 14, according to the CDC.

“We were surprised that there was this number,” Dr. Anne Schuchat, the deputy director at the CDC, told The New York Times. “If a number of them pan out, that’s much more than I was expecting.”

The Zika virus is probably linked to microcephaly. 

In the early days of the Zika virus outbreak, women — especially pregnant mothers — were in the spotlight of highest risk. The disease is almost certainly linked to microcephaly in infants, which causes babies to be born with dramatically undersized heads.

Governments in neighboring countries went as far as to suggest their citizens shouldn’t get pregnant while at risk of contracting the virus. In largely-Catholic, anti-contraceptive communities, this was a wholly unreasonable demand. Fortunately, Pope Francis gave Catholics in affected countries the OK to use contraception. Still, the CDC recommends that Zika patients and people exposed to the virus, especially those who are pregnant, refrain from sex or use condoms vigilantly for oral, vaginal, and anal sex. So if you or your partner have been exposed, especially if one of you is pregnant, it’s probably best to see a doctor and also practice safe sex.

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