He was concerned that Ebola’s persistence in survivors, who have no obvious symptoms of Ebola infection and so are often living and working normally and not kept in isolation as a symptomatic patient would be, means they are “a potential reservoir of the virus.” “It’s why men who have had Ebola and recovered are advised to abstain or wear condoms,” he noted.

The World Health Organization’s advice is that all male survivors should be tested three months after the onset of symptoms and then monthly until they know they have no risk of passing on the virus through their semen.

John Edmunds, an expert at the London School of Hygiene and Tropical Medicine, said that while the risk of transmission from survivors harboring the virus in their eye fluids and other organs “appears to be very low,” it still warrants attention. “With so many survivors in West Africa now, there is a risk that further outbreaks can be triggered, which is why authorities have to remain very vigilant,” he said.

Neuman said the likelihood of survivors spreading Ebola depends on how much of the virus is present in the blood. In Cafferkey’s case, he said, “if her body was able to control the virus once, the chances are she can do it twice.”