Concerned about the potential for sexual transmission of Ebola, international health officials are investigating new reports of suspected cases and beginning studies to determine how often and how long the virus remains active in semen. And, for now, they are warning Ebola survivors to practice protected sex indefinitely.

A few possible cases of sexual transmission surfaced during the peak of the epidemic in West Africa, but little follow-up occurred then. Now, with new infections down to around three dozen a week across the three affected countries, medical investigators are focused on two women who died of the disease last month — one in Liberia and the other in Sierra Leone.

The women had no known risk factors, but they did have male partners who had survived Ebola last year, suggesting that the virus could be active in semen longer than researchers had previously detected, and that it could spread through sexual activity.

In recent days, American and Liberian scientists have begun finding genomic evidence in support of sexual contact being the cause of the Liberian case, researchers said. Even though such transmission has never been confirmed and is assumed to be rare, it has raised concerns because of the potential impact on declaring the epidemic over.