Months after a male Ebola survivor tested negative for the disease, he transmitted the deadly virus to a female partner through unprotected sex, a genetic analysis revealed.

The Liberian woman, who became ill with the disease and died in March, is the first person known to contract the Ebola virus from sex, researchers reported this week in the New England Journal of Medicine. Typically, people contract Ebola from direct contact with the blood or other bodily fluids from a sick or recently deceased patient.

But experts knew that the Ebola virus can linger in patients after they’ve recovered. And they speculated that sexual transmission was possible. After the virus is cleared from a patient’s blood, it can turn up in semen and other fluids for weeks or months—as long as nine months, new data suggest.

This was the case for the male survivor, also Liberian, when he transmitted the virus to his partner. His blood tested negative for the virus 155 days before the pair had sex. But semen samples taken after the woman fell ill revealed he was still shedding the virus.

Genetic sequencing revealed that the two were infected with identical viruses, distinct from other versions of the Ebola virus circulating in Western Africa. The finding leaves little doubt of direct transmission between the survivor and his partner.

Still, sexual transmission may not pose a significant threat to public health. In an accompanying editorial, Armand Sprecher of Doctors Without Borders wrote that “sexual transmission remains a rare event,” noting that there are 17,000 survivors of the most recent Ebola outbreak. If sexual transmission were common, he wrote, there would be many more cases.

NEJM, 2015. DOI: 10.1056/NEJMoa1509773 (About DOIs).