Men can contract Zika through unprotected sex with other men who are infected with the virus, the Center for Disease Control and Prevention said Thursday, a finding that adds to experts’ understanding of how Zika can be transmitted.

The CDC said a case of sexual transmission that occurred in Dallas in January — and was reported in the mainstream media in early February — involved two men. It was only the second reported case of sexual transmission of Zika ever, and the first in which sexual spread was observed between two men.

While gay men who want to avoid Zika infection should take note, the lesson here applies to both men and women: Zika infection can be contracted through unprotected anal sex.

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Dr. John Brooks, from the CDC’s division of HIV/AIDS prevention, said the case doesn’t change the agency’s recommendations on how to avoid sexual spread of the virus, which were based on the assumption that various types of sex could transmit Zika. But it gives scientists an example to point to, he said.

“It’s not just theoretical. It’s happened,” said Brooks, who was one of the authors of the report.

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The new findings were published in the CDC’s Morbidity and Mortality Weekly Report. The authors are from the Dallas County Health Department and the CDC.

“The take-home message is you have to consider any kind of intimate contact between an infected person with Zika and a non-infected person as a potential risk situation, regardless of gender,” said Michael Osterholm, director of the Center for Infectious Diseases Research and Policy at the University of Minnesota.

Osterholm, who was not an author of the report on the Dallas case, noted it’s still not clear if the virus can be transmitted through kissing or oral sex, though the virus is found in both saliva and semen. “That’s difficult to tease out,” he said.

A report published Wednesday in the New England Journal of Medicine highlights that difficulty. Scientists from France reported on a case of sexual transmission of Zika from a man infected abroad to his female partner. The two had unprotected vaginal sex without ejaculation and oral sex with ejaculation.

Brooks said the issue needs more study, but he suspects transmission by saliva isn’t a regular feature with the Zika virus.

“Were saliva an important mode of transmission, my expectation would be that we would have seen the epidemiology of cases evolve differently,” he said. “We’re just not getting a signal.”

Since the Zika outbreaks in the Americas began, seven cases of sexual transmission of the virus have been observed in the United States. And five other countries — Argentina, Chile, France, Italy, and New Zealand — have reported sexual transmission cases.

The Dallas case involved a man who had been infected with the virus while traveling in Venezuela. Two days after he returned home, his symptoms developed. A week after his return, his longtime partner, who had not traveled outside the country, became ill.

The investigators tried to rule out the possibility that the second man might have been infected locally by a mosquito. Mosquito traps set up around the couple’s home collected some Culex mosquitoes, but no Aedes moquitoes, the type believed to be the main culprit in spread of Zika virus.

CDC officials have expressed surprise at how often sexual transmission cases have been seen in this outbreak. And the agency’s report noted that sexual spread “might contribute to more illness than was anticipated when the outbreak was first recognized.”

In areas where Zika is spread by mosquitoes, it’s virtually impossible for researchers to distinguish cases of sexual transmission from cases in which the virus was transmitted directly from mosquitoes. But this outbreak has made it easier to identify sexual transmission cases, as infected travelers have returned to areas where local spread is not occurring.

Such instances “offer unique and important opportunities to learn about this emerging mode of transmission and rapidly inform and refine interim prevention recommendations,” the authors wrote.