The transmission occurred between two Dallas men in January, indicating the virus can be spread through anal sex.

The Centers for Disease Control and Prevention has confirmed the first male-to-male sexual transmission of the Zika virus in the U.S.

The transmission occurred between two men in Dallas, the CDC announced last month. One man had traveled to Venezuela, and two days after returning home in January, he showed symptoms of the virus — fever, a rash on his face and upper body, and conjunctivitis, also known as pinkeye, which makes the eye pink and sore.

Before his symptoms developed, he had had anal sex with his partner, without a condom, and a week later, the partner exhibited similar symptoms, the CDC reports. Lab tests determined both men’s conditions were caused by Zika.

“The second man had had a Zika virus infection and the only way he could have gotten it was from his sexual partner,” CDC medical epidemiologist John Brooks, an author of the report, told NPR. The virus is also mosquito-borne, but it was too cold in Dallas in January for mosquitoes to survive, and the men were in a long-term monogamous relationship.

The men recovered quickly — the illness is usually mild, and many people infected with Zika don’t even have symptoms, according to the CDC. But it can cause brain damage in babies born to women infected with it during pregnancy, which has led the CDC to term the Zika outbreak a public health emergency. The CDC has documented five U.S. cases of male-to-female transmission, likely through vaginal sex, and the findings released today indicate it can be transmitted through anal sex as well.

In addition to the possibility of transmission through semen, a New England Journal of Medicine report on a case of male-to-female transmission says the virus may be transmitted “through other biologic fluids, such as pre-ejaculate secretions or saliva exchanged through deep kissing.” There have not been reports of Zika found in vaginal secretions, though.

The CDC is likely to recommend that all men who may have been exposed to Zika use condoms during sex, be it anal or vaginal, notes U.S. News and World Report. “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,” Brooks told NPR. “A woman who has anal sex could become infected with Zika virus.”

The findings also “highlight the need for clinicians to remain vigilant for and continue reporting any suspected cases of Zika virus infection to their state or local health departments, including suspected infections in symptomatic persons without travel history, but who report unprotected sexual contact with a person who has traveled to an area with active Zika virus transmission,” the CDC report emphasizes.

“We really owe a debt of gratitude to the two men who came forward as well as to the extraordinarily astute clinician,” Brooks told NPR. “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.”

So how worried should you be about the risk of Zika? Find out here.