Health officials announced Tuesday that they are investigating 14 new U.S. cases of possible sexual transmission of the Zika virus.
The virus was confirmed to be in blood samples from two women, using a method that detects pieces of the virus' genetic material, say doctors from the Centers for Disease Control and Prevention.
The women had not recently traveled to places where the virus is widely circulating, but they'd had sex with male partners who had recently returned from such travel and had Zika symptoms. The men, who are no longer sick, are now being tested for signs of the virus, too.
The other 12 possible cases — all women — have not been confirmed yet. Four tested positive in an initial test, but it can yield false positives. Doctors are waiting on results from a more definitive test to confirm Zika infection.
Capt. Jennifer McQuiston, Deputy Incident Manager for the CDC's Zika response, says she expects more cases of sexual transmission in the next few months.
"As doctors become more aware of the sexual transmission risk, they will begin to test more," she says, "and they will likely begin to find more."
Illness with the virus is usually mild and short-lived, but it's the possible connections to a neurological condition called Guillain-Barre syndrome and to a congenital condition known as microcephaly that are worrisome. The Centers for Disease Control and Prevention says "several" of the 14 women are pregnant.
The CDC continues to recommend that men who have been to an area where there is active Zika transmission use condoms during sex with a pregnant partner, or abstain from sex for the duration of her pregnancy.
It also recommends that travelers to regions where there is ongoing transmission use insect repellent for three weeks after they return to the U.S., to keep from being bitten by domestic mosquitoes and possibly passing along the virus.
There is no evidence at this point that women infected with Zika can transmit the virus to their sex partners, according to the CDC.
The best way to avoid getting Zika is to avoid mosquito bites.
"The CDC still feels that mosquito-borne transmission is the most common route for Zika," says McQuiston. "But we don't yet have mosquito-borne transmission [in the U.S.], so the sexual transmission cases are coming to our attention."
Earlier this month the CDC reported the current outbreak's first U.S. case of sexual transmission of the Zika virus. And Brian Foy, a biologist at Colorado State University who studies vector-borne illnesses, suspects he transmitted the Zika virus to his wife via sex in 2008, when he returned from a trip to Senegal. His lab has since partnered with researchers in Brazil to study sexual transmission of Zika virus.
In a place like Brazil, where there's active transmission going on, it's harder to disentangle who got Zika from a mosquito and who got Zika through sex, he says.
"There are mosquitoes biting people, and maybe there's sexual transmission," Foy says. "But how do you tell the difference?" The U.S. cases may help scientists figure out more about how the virus tends to move.
"This might be something that's happening much more often than anybody had previously suspected," he says.
Still, from the virus' perspective, it would take a lot of fortuitous occurrences for a man to pass it to a sexual partner: He would have to get infected with the virus first; he'd have to be sexually active; and he'd have to have sex with someone before his body cleared the virus.
"There are a lot of things that have to happen for sexual transmission to occur," Foy says. To get Zika from a mosquito, it just takes one bite from the wrong bug.
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