Inside the effort to track possible animal-to-human virus transmission

In National Geographic’s “Virus Hunters," now streaming, ABC News' James Longman follows a team in West Africa that tracks possible poachers who are likely exposed to bats’ urine, feces and blood.
6:41 | 11/13/20

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Transcript for Inside the effort to track possible animal-to-human virus transmission
Sxwr as covid-19 cases surge across America, leaders are imploring people to help stop the spread. Every single thing you do I'm tired, you're tired, but we must remain vigilant. We know that masks work. Reporter: Dave shutdowns have got the nation on edge. The country's leading infectious disease expert Dr. Fauci bringing a glimpse of hope. Help is really on the way. If you think of it metaphorically, the cavalry is coming here. Vaccines are going to have a major positive impact. It's going to be January, February, March. More and more and more people are going to be able to be vaccinated. If we could just hang in there, do the public health measures that we're talking about, we're going to get this under control, I promise you. Reporter: To curb an outbreak like this, scientists first have to learn how viruses like the coronavirus spread. So we set out to better understand how pandemics start and how they can be stopped. This is it. We've just got to Liberia. Chris has made it through as well. Reporter: I'm here in Liberia in west Africa with epidemiologist Chris golden. Liberia was ground zero for the ebola outbreak. And a place researchers are looking for the source of the next pandemic. If people are coming into contact with bats in this rain forest, they could be exposed to a new strain of ebola or another virus. The animals coming from this side -- Reporter: Moses is a ranger whose job it is to stop poachers. He his team are our first line of defense. You're standing at this abandoned mine. You can only see maybe two or three meters in before it's entirely black. But in the kind of haze, you can see the kind of decrepit old bits of previous mining industry. And overwhelmingly, a squawk of all these bats. You can see them right here. Hm, wow. So there's an entire colony in here. Yeah. And people want these bats, right, Chris? These are valuable? In many places these are a preferred food item, where people will pay almost a premium price. This would be a prime hunting ground. Reporter: Moses and his team find this trap which seems to suggest that humans have been inside. You have hunters who are staying in this cave night after night, being exposed to bat feces, bat urine, bat bites. There's actually horseshoe bats and a type of fruit bat. Horseshoe bats are known to be related, potentially, to covid and also to ebola. Oh, wow. Oh my god. Reporter: We go further and further back into the depths of the cave, and Moses looks down, and he actually sees a whole host of dead bats on the ground. It could be that these bats have died of some kind of virus. It really does feel like we're in a front line of something unknown. Young bats should not be buying, basically. Not like this. And I think obviously the issue is, if there's a poacher in here, there's a dead bat, there's blood or er, right? So that's a problem. Yes, there's probably some sort of skis, exactly. Bats were known to be the reservoir of ebola. It transferred to other types of animals and was then transmitted to humans. This is the exact way a new deadly virus could start. Right now we're headed into the central market of Monrovia. Whatever the poachers kill in the forest they come here to sell We have to figure out where is the bush meat going and who are the people helping it along its way? Because they are key figures in what might lead to the next deadly virus. Reporter: We know the bush meat trade played a role in the ebola outbreak here. Rangers like Jimmy are frontline defenders in the battle to stop a new outbreak. There was a larger bush meat market before ebola. And so during ebola, it almost like ceased, all right? And then after the ebola, people started going back to the market. The only action is thorough awareness like we are doing now and law enforcement. This is one of the most hectic markets I think I've ever been to in my life. It's incredible how much B meat is here. I haven't seen anything like this. You haven't? There are more. Before you would have seen the whole place like that. The case is people need to feed their families, there's a huge demand for it, and this is just one market. What worries me here is that this market normous demand. That will then drive more and more people to go into the forest, increasing human/wildlife interaction. Almost 75% of all emerging infectious diseases globally come from an animal reservoir and an animal host. When I think about this from a planetary health perspective, you really understand that all of this is connected. Nd the world, new outbreaks are happening more and more freq. In places where humans and animals are coming into closer and closer contact. At a certain point, nature is forced to react. And this could really be conceptualized a series of dominos that are all falling down in a line. Each domino represents a touch point of exposure to a new virus hiding in some corner of the Earth. Removing just one of those dominos could stop the next pandemic. Our thanks to James. "Virus hunters" is available right now on the national geographic and TV everywhere apps and on demand.

This transcript has been automatically generated and may not be 100% accurate.

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