In January, a man returned home to Dallas after a weeklong trip to Venezuela. Two days later he got a fever, a rash on his upper body and face, and pinkeye.

He was fine within three days. But about a week later, his partner developed similar symptoms — a rash, pinkeye and some joint pain in his hands and feet.

He went to the doctor’s office, and a clever physician assistant thought of Zika virus.

The day before the traveler’s symptoms set in, he had had anal sex with his partner. They’d been together for more than 10 years. They’d had sex again a day after the man recovered. The traveler hadn’t noticed anything unusual about his urine or sperm, and the couple didn’t use condoms.

Suspecting sexual transmission, the physician assistant collected samples of blood from both men, and later collected semen, urine and saliva as well. Though the virus itself was no longer conclusively detectable in either of the men’s samples, a more thorough test showed that the man who had not traveled had been infected with Zika.

The test results did not clearly show whether the traveler had had Zika or a certain type of dengue virus or both. But the researchers were able to rule out that the man who did not travel could have gotten it from a mosquito — it was too cold in Dallas in January for Aedes aegypti larvae to survive, and traps laid around the couple’s home only brought in a different species of mosquito not known to carry Zika.

“The second man had had a Zika virus infection and the only way he could have gotten it was from his sexual partner,” says Dr. John Brooks, a medical epidemiologist with the Centers for Disease Control and Prevention and an author on the case report published Thursday by the CDC.

“We know that most STIs can be transmitted through anal sex and oral sex as well as vaginal sex,” says Brooks. So the fact that Zika can, too, is not a surprise, he says.

The important thing, Brooks says, is that the finding is not just relevant to gay men.

“Anal sex is practiced not only by men who have sex with men, but also by heterosexual couples, and so it’s important to consider not the person’s sexual preference but the activity in which they’re engaging,” says Brooks. “A woman who has anal sex could become infected with Zika virus.”

The big question now is how long the virus can last in the semen of an infected man, and therefore how long he might be capable of infecting someone else through sex. It’s also unclear how likely it is that a man infected with Zika will shed it in his semen, and if that only happens with people who show symptoms of the virus. In the six reported cases, all the men with detectable virus in their semen had shown symptoms, but most people who get the Zika virus are asymptomatic.

During a previous outbreak, the virus was detected in the semen of one man two months after his fever had set in. In a more recent case, researchers found the virus in the semen of a French traveler two weeks after he’d been sick (there was a lot more virus in semen than there was in blood or urine, leading them to wonder if the virus can replicate in the male genital tract).

And a case was reported Wednesday in the New England Journal of Medicine of a woman in Paris who contracted Zika after having sex with a partner who had traveled to Brazil. Researchers isolated the virus from an infected man’s semen 24 days after his symptoms started. The two had had oral sex as well as vaginal sex without ejaculation.

The authors write that the data “support the hypothesis of sexual transmission (either oral or vaginal)” of the Zika virus. “We cannot rule out the possibility that transmission occurred not through semen but through other biologic fluids, such as pre-ejaculate secretions or saliva exchanged through deep kissing,” they write.

There is no evidence that vaginal secretions of an infected woman can contain Zika virus.

“We really owe a debt of gratitude to the two men who came forward as well as to the extraordinarily astute clinician,” says the CDC’s Brooks. “We really encourage clinicians and public health officials to report cases of suspected sexual transmission to their local health departments so we can get an idea of the biology of the cases.”

Dr. Paul Mead, a CDC epidemiologist, says he’s working with state and local public health labs to get in touch with U.S. travelers who have tested positive for Zika. He aims to enroll 250 men in a six-month trial to understand how long the virus persists in semen and urine. There’s a similar trial in the works in Puerto Rico.

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