RACHEL MARTIN, HOST:
The World Health Organization is in the midst of an experimental campaign to vaccinate tens of thousands of people in Congo against Ebola. The country is battling a new outbreak of the disease there. Also in the news - an outbreak of a deadly disease called Nipah in India. These are just some of the latest viruses to raise alarms around the world. Two years ago, it was Zika - before that, bird flu. Health officials say all these viruses have the potential to kill millions, and yet they haven't. NPR's Michaeleen Doucleff is here to explain why.
MICHAELEEN DOUCLEFF, BYLINE: Hi, Rachel.
MARTIN: So what's going on? Why are there so many false alarms, if we can call them that?
DOUCLEFF: Yeah, so first off, I don't want to minimize the power of these diseases. These are incredibly destructive outbreaks, even when they're small or just restricted to a small area. For instance, the Ebola outbreak in West Africa killed more than 11,000 people. But for viruses to turn into pandemics that wipe out millions of people, they need two things - a high mortality rate, and they need to spread very easily. And it turns out, for viruses, this is really hard. For instance, Ebola - it's very deadly but doesn't actually spread very quickly or well. Same goes for Nipah, the virus that just cropped up in southern India. It kills up to 70 percent of people infected, but it also doesn't spread very well.
MARTIN: What about the viruses that do spread really easily, like Zika, right? That was the thing with Zika. It spread explosively, especially around South America and Central America.
DOUCLEFF: Yeah, absolutely. It's super contagious. But for the vast majority of people, Zika is a mild illness. Many people don't even know they have it at all. And so often, this is the case. A virus is either super contagious or really deadly, and the combination is super rare.
DOUCLEFF: And this is actually why Zika has basically disappeared. So many people were infected and survived back in 2016 that a huge chunk of the population now is immune, and the case numbers have just plummeted.
MARTIN: So if it's really hard for these viruses to turn into actual deadly pandemics, why do we keep raising the alarms? Why are we so concerned about it?
DOUCLEFF: The problem is - is that viruses can change, and they can change really quickly. A good example of this is the 1918 flu. Have you heard of that?
MARTIN: Right. I mean, yeah. This is one of the worst pandemics in history.
DOUCLEFF: Yeah. It killed, like, 50 to 100 million people - and really quickly. But this pandemic wasn't a total surprise. Historical records suggest that there had been other outbreaks and even pandemics decades before of a similar but less deadly flu virus. And then all of a sudden, the virus started killing at a much higher rate - like, five to 20 times. And no one is really sure why, but there was this massive change in the virus's genes, and these changes have been so powerful that many of the flu viruses that we see today are direct descendants of the 1918 flu.
DOUCLEFF: So here's the thing. Every time a virus causes an outbreak, every time we have an Ebola outbreak or a Nipah outbreak, it gives the virus a chance to mutate and evolve in people, to figure out how to thrive in people and just maybe find that rare combination of contagious and deadly. So it's a bit like rolling the dice. If you roll dice enough times, eventually, you might get snake eyes.
MARTIN: OK, so you're saying that even though the chance of a pandemic is really slim, anytime there's an outbreak of one of these diseases, it might be getting stronger.
DOUCLEFF: Yeah. And that's why a lot of people are working hard to stop the outbreaks and figure out how to prevent a pandemic.
MARTIN: All right, Michaeleen Doucleff, global health correspondent for NPR News. Thanks so much.
DOUCLEFF: Thank you. Transcript provided by NPR, Copyright NPR.