Welcome to the bat cave. No, we're not talking about the secret headquarters of a superhero.
This is Gomantong — an ancient cave carved out of 20 million-year-old limestone in the middle of the Borneo rain forest in Malaysia. It's part of a vast network of tunnels and caverns. And it's the perfect hideout for bats.
Up at the top are millions of bats. Literally millions. They hang upside down all day long from the cave's ceiling, sleeping and pooping.
"Oh dear! We've been dripped on," says Mike Lindley-Jones, a doctor from Australia, as some liquid falls on his head. "Is this bat urine?"
"No, it's just water," says Jimmy Lee, with the U.S.-based nonprofit EcoHealth Alliance.
Lee is guiding us through the cave, along a wooden boardwalk. Suddenly Lindley-Jones grabs the handrail covered in guano.
"Don't touch your face!" Lee warns, because inside that bat excrement could be something potentially dangerous.
"That's why I'm wearing a mask," says disease ecologist Kevin Olival, who's also with EcoHealth Alliance.
We're all wearing masks. Because bats tend to carry lots of viruses. Some of them are harmful to us. Bats are vital for keeping the rain forest alive — they are key pollinators for more than 500 kinds of plants. But as people spread out around the globe, we're increasingly coming into contact with bats — and the viruses they carry. Right here, we're using one of their favorite hideouts as a tourist attraction.
"Visiting beautiful places like this inspires people to protect tropical ecosystems and the species that live here," Olival says. "At the same time, we need to recognize that there may be potential health risks when people and wildlife come together, and that's why we're working to understand and limit those risks."
A few years ago, Olival, Lee and their colleagues went hunting for viruses around this cave, in partnership with the Sabah Wildlife Department and the Danau Girang Field Centre. And they couldn't believe what they found.
"We found 48 new viruses in the surrounding forest," Olival says, "including a virus related to SARS in bats that roost in the cave."
Olival and his colleagues don't know yet if these new viruses can infect people. They are related to viruses that do. But SARS-like viruses aren't something to mess around with.
In 2003, SARS scared the world. The deadly virus emerged in China and spread to Singapore and Hong Kong. Then hopped on a plane to Toronto and cities in the U.S. People panicked. They thought this was the big one — a pandemic that could kill millions. Luckily, health authorities stopped it in time.
Now this SARS-like virus could be above our heads. Dripping from the walls. Even circulating through the air.
Playing the viral lottery
Historically, it's been difficult for scientists to pinpoint where a deadly outbreak begins. For the recent Ebola outbreak in West Africa, the culprit was possibly a hole in a tree where toddlers liked to play and may have come into contact with infected bats. SARS very likely came from a civet cat, caged at a meat market.
Like these places, bat caves around the world have the two essential ingredients to brew up a new plague: intrusion of people into a previously hidden habitat and viruses circulating around.
In this case, the Gomantong Cave is an ecotourist hot spot. People come from all over the world to see the cave — and the orangutans and pygmy elephants in the surrounding rain forest. Tourism has helped protect this stretch of rain forest and these animals. If it weren't for tourism, there's a good chance much of this forest would've been cut down and converted into palm oil plantations. One look at this area on Google Earth, and you'll see that there is just a narrow strip of forest left.
The cave also provides another source of income for families that live nearby. Up high along the cave's walls, swiftlet birds zoom in and out of their nests. The nests are made from the birds' saliva and are a delicacy in Chinese cuisine. So each year, men climb on rope ladders along the walls to collect the nests and sell them.
Below the birds and their nests, another creature reigns supreme.
"That wall is plastered with cockroaches," says Sammy Whitaker, a young boy from New Zealand, touring the cave with his parents.
Cockroaches are everywhere in the cave. They're all over the floor. On our shoes. There are so many that we can even hear them.
But the cockroaches aren't what worries scientists about the cave. It's what the critters are eating: bat poop.
In the middle of the cave is a giant mountain of bat guano. It looks like black snow that's 8 to 10 feet deep. It smells awful, like moldy socks mixed with rotting cheese.
And some of that guano might contain this new SARS-like virus, Olival says.
But the risk any one tourist faces is low. So low, Olival says, that for a virus to infect a tourist, it would be like the virus winning the lottery. Only some bats in the cave have the virus. So only a few of the guano pellets are likely contaminated. And then, the tourist has to touch those contaminated pellets and rub his eyes or mouth to get infected. That's another long shot.
But here's thing about lotteries: There's always a winner. A big winner. Scientists worry it's the same thing with virus-filled bat caves.
"It's a numbers game," Olival says.
Because there are so many bats in caves around the world and so many people now visiting them, eventually it's going to happen in some cave at some moment.
In fact, Olival says, it already has.
In 2007, a woman from Colorado took a trip to Uganda, where she visited a bat cave, kind of like the Gomantong Cave.
A few days after she returned home, she got very sick. Eventually the woman was diagnosed with Marburg virus — a close relative of Ebola.
She recovered. And scientists think they know how she got the virus: She put her hand on rocks covered in bat guano.
Fortunately, the woman didn't spread the disease to anyone else. But it could be just a matter of time before another tourist does.
Tourism is one of the fastest-growing economic sectors in the world. In 2015, people took more than 1.2 billion international trips as tourists. Tourism helped Zika spread around the Americas and brought SARS to North America.
A risky cave souvenir
Just as we're leaving this cave in Borneo, we run into two more tourists — a father-son pair from the U.S., Anthony and Joe Caravello.
They've just finished the cave tour, and Anthony notices something on his dad's hat.
"You've got a piece of guano on your hat," Anthony says. "Actually, you've got a few pieces."
"Just what I wanted," Joe says with a chuckle. "Maybe I could take it home and show it off." Because soon Joe will hop on a plane and head back home to Florida.
Like most people touring the cave, Joe wasn't told about the potential risk. Because why scare away business when the risk is infinitesimally small for any one tourist?
But eventually someone, somewhere, won't be so lucky. And the impact of that could be global.
What do you want to know about pandemics? Share your questions here. Our global health team will answer some of them in an upcoming story.
DAVID GREENE, HOST:
On the island of Borneo in Malaysia, there is a cave that tourists from all over the world love to come to visit. It is called Gomantong. But this cave, it turns out, has a secret. It could have all the elements needed to start a pandemic.
RACHEL MARTIN, HOST:
The world is seeing more disease outbreaks than ever before - Ebola, Zika, bird flu - and the speed at which new viruses are appearing is increasing. This month, NPR is exploring why.
GREENE: And today, NPR's Michaeleen Doucleff takes us deep inside this cave to explain one reason why - it's the exotic vacations we like to take.
MICHAELEEN DOUCLEFF, BYLINE: If you want to see for yourself a place where deadly viruses could be hiding, you first need to buy a ticket.
GRAHAM WHITAKER: Three adults and one child.
DOUCLEFF: I'm in line with Graham Whitaker, his wife Jenny and son Sam. They're visiting from New Zealand. And we're heading into the mouth of the cave.
JENNY WHITAKER: Have you got a head torch Sammy could borrow, please?
DOUCLEFF: When you walk inside, it's breathtaking.
J. WHITAKER: My goodness.
G. WHITAKER: Fantastic.
DOUCLEFF: The cave is gorgeous. It looks like a cathedral. Forty, 50 feet high with this light streaming in from the side - it really is spectacular.
J. WHITAKER: How amazing.
DOUCLEFF: So we're going deeper in to the cave now.
J. WHITAKER: Be careful.
G. WHITAKER: It's slippery.
DOUCLEFF: Whoa, almost just slipped (laughter).
When Sammy shines his flashlight on the wall, we quickly realize we're not alone.
SAM WHITAKER: Whoa.
J. WHITAKER: Whoa.
G. WHITAKER: Oh, my word. That wall is plastered with cockroaches.
DOUCLEFF: They're all over this banister.
SAM: They're everywhere.
DOUCLEFF: Because in the middle of the cave, there's an enormous mountain of their favorite food.
G. WHITAKER: Bat poo, basically just solid bat poo.
DOUCLEFF: Eight, maybe 10 feet deep, and the cockroaches love it. They feast on bat guano morning, noon and night. And boy, does it smell bad.
SAM: I'm trying not to breathe through my nose.
DOUCLEFF: Because of the smell?
SAM: Yeah - whoa.
J. WHITAKER: Oh, look, Graham, there's bats there.
DOUCLEFF: Right above us right now in the dark hole at the top.
That's the world of the bats. And there are millions, literally millions, of bats in these caves. And they just hang up there during the day, sleep and poop.
MIKE LINDLEY-JONES: We've been dripped on. Is this bat urine?
JIMMY LEE: It's just water.
LINDLEY-JONES: Just water.
DOUCLEFF: That's Mike Lindley-Jones. He's over from Australia. And he suddenly grabs the handrail covered in guano.
LEE: Don't touch your face.
DOUCLEFF: Don't touch your face, he's warned, because there may be something hidden in the bat guano, or even floating around in the air, something invisible - viruses.
KEVIN OLIVAL: Viruses are pretty hard to see.
DOUCLEFF: That's Kevin Olival. He's a virus-hunter with the New York-based EcoHealth Alliance. And this cave has him worried.
OLIVAL: That's why I'm wearing this mask.
DOUCLEFF: We're all wearing masks. That's because viruses love it here. They love bats. Some of our deadliest viruses hang out in bats - rabies, MERS, Nipah, Hendra, Marburg. And when those viruses spread from bats into people, they can cause a lot of damage. That's likely how the recent Ebola outbreak in West Africa got started. Olival says bats have had these viruses for a long, long time.
OLIVAL: Tens of thousands of years, if not longer.
DOUCLEFF: But now people all over the world are coming into contact with these bats at a faster rate than ever before. Bats are vital for keeping the rainforest alive. But we're increasingly cutting down rainforest and destroying the bats homes, hunting them down for food. And right here, we're using their favorite hideout as a tourist attraction. Last year, Olival's team went looking for new viruses in this cave and the surrounding forest. They couldn't believe what they found.
OLIVAL: We found about 48 new viruses and about 15 known viruses.
DOUCLEFF: And are any of those viruses dangerous to people?
OLIVAL: We found a lot of viruses we know nothing about. But a few of the viruses we found are related to SARS.
DOUCLEFF: SARS, a deadly virus that scared the world in 2003. It started in China and spread through Asia, hopped on a plane to the U.S. People panicked. They thought this was the big one, a pandemic that could kill millions. Luckily, health authorities stopped it in time. Now this new cousin of SARS could be above our heads in the bats.
(SOUNDBITE OF BAT DETECTOR BEEPING)
OLIVAL: There you go. You just heard one.
DOUCLEFF: What is this device?
OLIVAL: This is a bat detector, which is a little box that allows you to hear what humans can't hear, which is the ultrasound that bats emit when they fly around. They're all around us. You could see them coming off the walls, actually.
DOUCLEFF: So the bats are hanging above us, hanging upside down above us, pooping out onto the floor of this cave. So some of that bat poop must contain this new virus.
OLIVAL: Yeah. I mean, a few of those pellets (laughter) probably do. But, you know, it's a very, very, very low risk.
DOUCLEFF: Such a low risk, Olival says, that for a virus to infect a tourist, it would be like the virus winning the lottery. Only some bats in the cave have the virus, so the chance that this bat poop is contaminated is low and then the tourist has to touch that contaminated poop, rub his eyes or mouth and get infected. That's another long shot. But here's the thing about lotteries, there's always a winner. And scientists worry it's the same thing with virus-filled bat caves.
OLIVAL: It's really a numbers game.
DOUCLEFF: Because there's so many bats up there and so many people in the world, eventually, it's going to happen - in some cave, at some moment.
OLIVAL: Yeah, it already has.
DOUCLEFF: Already has (laughter)?
DOUCLEFF: Back in 2008, a woman from Colorado took a trip to Uganda where she visited a bat cave, kind of like the one here. A few days after she got home, she got really sick. Eventually, she was diagnosed with what's called Marburg virus, a close relative to Ebola. She recovered. And she thinks she knows how she got the virus. She put her hand on a rock covered in bat guano. Fortunately, the woman didn't spread the disease to anyone else. But Olival says it's just a matter of time before another tourist does.
Tourism is one of the fastest growing economic sectors in the world. Last year, people took more than 1.2 billion international trips as tourists. And the consequences of any one of those trips could be big. Tourism helped spread Zika around the Americas, SARS to North America. And it's one of the main ways the flu gets around the world.
As we're leaving this cave in Borneo, we run into two more tourist. It's a father-son pair from the States.
ANTHONY CARAVELLO: Anthony Caravello.
JOE CARAVELLO: Joe Caravello.
DOUCLEFF: Nice to meet you. And you're from...
J. CARAVELLO: From Florida.
DOUCLEFF: They just finished inside the cave when Anthony notices something on his dad's hat.
A. CARAVELLO: You've got a piece of guano on your hat.
J. CARAVELLO: (Laughter) Just what I wanted.
A. CARAVELLO: You've actually got a few pieces.
J. CARAVELLO: Well, wonderful. Maybe I can bring it home and show it off, right?
DOUCLEFF: In a few days, Joe says he's hopping on a plane and heading back home to Florida. Like most people touring the cave, Joe wasn't told about the potential risk because why scare away business when the risk is infinitesimally small for one tourist? But eventually, someone, somewhere won't be so lucky. And the impact of that could be global.
Michaeleen Doucleff, NPR News.
(SOUNDBITE OF EDAMAME'S "SOME WHISPERS")
GREENE: And you should check out this animation of how an animal virus transforms into a deadly human virus at npr.org/pandemic Transcript provided by NPR, Copyright NPR.