Just when health officials think the Ebola outbreak is over in West Africa, the virus pops up again seemingly out of the blue. It's happened at least five times so far.
Now scientists are starting to figure out why: The virus can lie dormant in a survivor for more than year and then re-emerge to infect others.
It's called a "persistent infection." It's rare. But it has played a big role in keeping Ebola around in Liberia, an international team of scientists reports Friday in the journal Science Advances.
"We believe that most, if not all, the clusters of new Ebola cases have come from [persistent infections in] survivors, but sometimes it's very hard to determine that with certainty," says Dr. Thomas Frieden, who directs the Centers for Disease Control and Prevention in the U.S.
Typically when a person recovers from Ebola, the immune system clears out the virus from the blood and other tissues and the person isn't contagious anymore.
But sometimes, the virus finds its way into parts of the body where the immune system can't reach it — such as inside the eyes, spinal fluid, breast milk and testes.
That last one is the big problem, says Dr. Daniel Bausch, an infectious disease expert at Tulane University. "Sexual transmission is our No. 1 concern," he says.
Scientists have known for years that Ebola can linger in the semen of men who've recovered — and that sexual transmission was possible. But a cluster of cases in Guinea this past March suggest semen can stay infectious for more than a year, far longer than previously thought.
So far, sexual transmission has likely caused at least two of the Ebola flare-ups recorded in Guinea and Liberia — which, in some ways, is a relief, Bausch says.
"Of course, I'm never going to say that sexual transmission is a good thing," he says. "But it's kind of comforting that these [flare-ups] are from a mode of transmission that we understand more and more about."
So what about the other flare-ups? That's where the new study comes in.
When Ebola first returned to Liberia last June, epidemiologists were stumped. They couldn't tell where the seven infected people had caught the virus.
So Jason Ladner and his colleagues at the U.S. Army Medical Research Institute of Infectious Diseases went hunting for clues. They isolated Ebola from the patients' blood and sequenced the genomes.
"People were saying this is probably the most important Ebola genome sequence that has been generated thus far," Ladner says, because they hoped it would tell them how Ebola had returned to Liberia.
When Ladner and his team got the results, they were quite surprised. The sequence for each patient's virus almost perfectly matched the one circulating in Liberia nearly a year earlier. During that time, the virus should have mutated. But it didn't. It was like the virus was frozen in time.
That meant the new cases in Liberia came from a dormant virus, hidden inside a survivor for more than a year.
Ladner and his team aren't sure who that survivor is. They have some hints that the flare-up might have begun when a woman passed the virus on to a man, perhaps through sexual contact — which would be a first. So far, only male-to-female transmission has been documented among survivors.
So that's the next medical question to probe — can sexual transmission go both ways?