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Until this week, most people living north of Mexico had the luxury of watching the Zika virus do its worst from a distance—while alarming, the outbreak felt remote. After all, the only Americans catching the virus were those who had the misfortune of being bitten by an Aedes mosquito while visiting an infected country.

That peace of mind was shattered on Tuesday when officials in Dallas reported the outbreak’s first known case of sexual transmission in the United States. We now know that someone contracted the virus after having sex with an infected patient in Texas. We no longer need mosquitoes to spread the scourge—we can do it all on our own.


But what does it really mean for Zika to be loose in the country as a “sexually transmitted disease”? Do people need to start asking their Tinder dates if they’ve recently visited one of the 26 infected countries?

While strikingly little is known about Zika—scientists are furiously working to learn more about everything from transmission to treatment—we do know that the virus can spread through sperm, and that individuals who plan on having sex with people who may be at risk should definitely wear a condom.


First, the basics on Zika. The virus is spread mainly through mosquitoes, and only about one in five people who contract it will become ill, according to the Centers for Disease Control and Prevention—showing symptoms including fever, rash, joint pain, and pink eye. Hospitalization and death are very rare. Some researchers are concerned that it could lead to Guillain-Barré syndrome, a rare neural disorder in which a person’s immune system damages the body’s nerve cells, but the evidence of this link is tenuous.

"I think there's a misunderstanding that this virus is fatal and will decimate society," says Karin Nielsen, a professor of clinical pediatrics in the division of infectious diseases at UCLA, who is also part of a team of researchers from the Fiocruz Institute in Rio de Janeiro investigating the epidemic. "But really, it causes a benign illness from which most people recover."

However, the Zika virus may pose an urgent danger to pregnant women, thanks to its potential to cause a devastating birth defect called microcephaly, marked by a shrunken head and incomplete brain development. The key word here is potential—Vox’s Julia Belluz offers a terrific breakdown of just how few cases have been confirmed. Yet the risk has generated enough concern that the World Health Organization declared the virus’ spread a “public health emergency” on Monday. Health officials in El Salvador have even advised women not to get pregnant until 2018.


The fact that Zika can be spread sexually is “no surprise,” CDC director Thomas Frieden told NBC News on Tuesday. Back in 2008, an American scientist who contracted the virus after visiting Senegal is believed to have passed the virus on to his wife. And the CDC explicitly lists sexual contact—as well as blood transfusion—as possible transmission methods on its website, thanks to previous (albeit rare) cases in which the virus was spread in these ways.

"The bigger concern,” Frieden explains, “is if a man comes back, gets sick with Zika, and then has sexual intercourse with a woman who is pregnant or may be pregnant, there is the theoretical risk that that woman could then become infected with Zika and her fetus could then potentially be infected."


Which is why, on Wednesday, the CDC updated its guidelines for pregnant women whose partners may have come into contact with the virus. “Until we know more, if your male sexual partner has traveled to or lives in an area with active Zika virus transmission, you should abstain from sex or use condoms the right way every time you have vaginal, anal, and oral sex for the duration of the pregnancy,” state the new CDC guidelines.

Another scenario? An infected man could, theoretically, impregnate a woman with a child at risk for microcephaly. This possibility adds a scary twist to the virus’ potential for wreaking havoc.

While the belief that only a fifth of people who contract Zika become sick may seem like a relatively small contingent, it's unclear if individuals without symptoms can spread the virus. (Dallas health officials have not revealed whether the infected traveler showed symptoms of the virus.) In the case of sexually transmitted viruses such as HPV and herpes, for example, the illnesses can be spread by people who are asymptomatic—but we just don't know enough about Zika's human-to-human transmission yet to understand how it spreads.


"To transmit a virus, you have to be viremic," explains Nielson, meaning the virus has to be in your blood. While the viremic window for the Zika virus is roughly 3 to 12 days, a person may not exhibit any symptoms and still be viremic, so theoretically, he or she might pass it on.

As for how long the virus can stay active in semen—doctors just don’t know. “The virus is in the blood for about a week. How long it would remain in the semen is something that needs to be studied and we're working on that now,” Frieden told CNN.

Stephen Higgs, an infectious disease expert and director of Kansas State's Biosecurity Research Institute, added that it’s challenging for scientists to track its spread, in part because, "If people do not have symptoms, then typically nobody collects samples to look for virus in the blood because they do not go to a physician.”


But Higgs points out another scary unknown as well: "There is also the unanswered question of whether the virus might persist in different tissues after the person has recovered." While Zika isn't a virus that is known for sticking around in the body, it's unclear if it could.

Meanwhile, in a paper published in May 2015 that sought to evaluate the likelihood of Zika being spread through blood transfusions, the authors expressed concern about how little is known about the virus’ behaviors, noting that “the main problem in the prevention of transfusion transmission is the high rate of asymptomatic infections and the mild disease that can go unnoticed."

The TL;DR version of Zika as an STD? People who don't know they have the illness could potentially spread it. Which begs the question of whether women in heavily infected areas should avoid having sex altogether—a question Nielson says has come up in conversations about how to handle the outbreak in Brazil.


Neilson says she agrees that women in these areas should try to avoid getting pregnant. While the pathogenesis has not been determined between microcephaly and Zika, she says, “there's reason to believe it's happening."

However, abstaining from sex as a means of preventing transmission would be pretty pointless, she says—there are so many mosquitoes around in the affected countries, everyone is at risk for contracting the disease.

In this country, the CDC says it will continue to update its information on sexual transmission “with a focus on the male sexual partners of women who are or who may be pregnant.” And in the meantime, as with preventing any sexually transmitted affliction, if you plan to have sex with someone who’s recently visited an infected country, the guidance is simple: Use a condom.


Taryn Hillin is Fusion's love and sex writer, with a large focus on the science of relationships. She also loves dogs, Bourbon barrel-aged beers and popcorn — not necessarily in that order.