DAVID GREENE, HOST:
You know, if we remember back to when Ebola was ravaging West Africa, this movement seemed so far away - the day we can declare West Africa Ebola-free. The World Health Organization made that announcement today, now that Liberia has gone 42 days without a new case. And so there is hope after an epidemic that claimed almost 12,000 lives in West Africa, but NPR's Nurith Aizenman reports there is still reason for caution.
NURITH AIZENMAN, BYLINE: Dr. Daniel Bausch is a leading scientific expert on Ebola for the World Health Organization. He also worked on the frontlines during some of the darkest moments of the West Africa outbreak. But while he's pleased about today's news, he's not exactly doing cartwheels.
DANIEL BAUSCH: You know, it's victory, but we need to be vigilant.
AIZENMAN: That's because today's announcement only means the Ebola virus isn't being actively transmitted from one person to another. It doesn't mean the virus has been eradicated from West Africa. For one thing, it lingers in the bodies of some of the more than 17,000 people who survived infection.
BAUSCH: Some people can maintain the virus in a few places where it's just harder for or takes longer for the system to get into and clean that place out - places like the semen in men and the internal contents or fluids of the eye, around your brain and your spinal cord.
AIZENMAN: Bausch says it's still unclear how long it takes for the virus to get cleaned out. And...
BAUSCH: While it's being cleaned out, there can potentially be transmission from sexual transmission from men. Or we do have some evidence that a few people here and there can get sick again with Ebola from that virus that they maintain.
AIZENMAN: Most common seems to be an eye infection in which the Ebola virus is basically trapped inside the eye, so it can't be transmitted to other people, but there's also the example of Scottish nurse Pauline Cafferkey. Months after she survived the Ebola disease, a virus in her cerebral spinal fluid caused her to develop meningitis, and some Ebola virus did get back into her bloodstream. Bausch says in cases like that...
BAUSCH: We think it's extremely rare, but there's the potential that that virus could come out and be transmitted to another person.
AIZENMAN: Then there's the risk of a totally new chain of infection. Ebola outbreaks are sparked when the virus jumps from animals - most likely bats - to humans.
BAUSCH: The bat that presumably introduced this virus into a young child in Guinea in 2013 was unlikely to be the only bat in the region that is infected. So we do have the risk of re-introduction from the wild, although we think that those events are rare.
AIZENMAN: All of which means it's likely we'll see at least a few more Ebola cases in West Africa this coming year. Bausch says as long as we jump on them quickly, they won't balloon into another epidemic. But we can't be complacent, he says. There is still hard work ahead. Nurith Aizenman, NPR News.