By day, some of the most dangerous animals in the world lurk deep inside this cave. Come night, the tiny fruit bats whoosh out, tens of thousands of them at a time, filling the air with their high-pitched chirping before disappearing into the black sky.

The bats carry the deadly Marburg virus, as fearsome and mysterious as its cousin Ebola. Scientists know that the virus starts in these animals, and they know that when it spreads to humans it is lethal — Marburg kills up to 9 in 10 of its victims, sometimes within a week. But they don’t know much about what happens in between.

That’s where the bats come in. No one is sure where they go each night. So a team of scientists from the U.S. Centers for Disease Control and Prevention traveled here to track their movements in the hopes that spying on their nightly escapades could help prevent the spread of one of the world’s most dreaded diseases. Because there is a close relationship between Marburg and Ebola, the scientists are also hopeful that progress on one virus could help solve the puzzle of the other.

Their task is to glue tiny GPS trackers on the backs of 20 bats so they can follow their movements.

Jonathan Towner, a scientist with the Centers for Disease Control and Prevention, climbs out of a mining cave in Uganda. The Marburg virus claimed the life of one miner and infected several others working there in 2007.

“We want to know where they’re going on a nightly basis,” said Jonathan Towner, 52, who heads a CDC team that specializes in how these deadly viruses are spread. If the animals are feeding on particular fruit trees, that information could identify communities most at risk and help prevent future outbreaks. “It’s much easier to put together a picture and say to local authorities, ‘Look, this could be potentially how the virus is spread, this is what the bats are doing.’ ”

U.S. officials are so concerned about Marburg becoming a global threat that the CDC is seeking funding from the Pentagon’s Defense Threat Reduction Agency to cover the cost of the bat trackers, which are about $1,000 each. The CDC is hoping to track more of these Rousettus aegyptiacus bats in several other caves in Uganda.

Marburg’s potential to spread was made clear a decade ago when a pair of tourists on separate trips walked into the cave looking for ad­ven­ture and walked out with the virus. A Dutch woman died 13 days after her visit. The other visitor, an American woman named Michelle Barnes, survived after a long, painful illness. The cave was closed to tourists in 2008.

Marburg was first identified in 1967 when a shipment of infected African green monkeys from Uganda was sent to laboratories in Marburg and Frankfurt, Germany, and Belgrade, in then-Yugoslavia. Seven lab workers died within about a week. Since then, a dozen outbreaks have been reported, killing hundreds of people. Most took place near bat-infested caves or mines, including one last fall along Uganda’s eastern border with Kenya. Of four family members sickened, only one survived.

For the CDC scientists, success hinges on getting the tracking units, which are half the size of a pen cap, to stay on a bat body that is only about six inches long. A practice run in Atlanta with the same device on the same kind of bat in a special CDC laboratory failed. Trackers slipped off or were chewed by the bats.

“I have no idea how well this is going to go because it’s the first time we’ve tried it,” Towner said. “It could end up in total flames.”

‘Bring us back some data’

Well before you see the bats — about 50,000 live in the cave — you hear their squeaks and chatter and smell the ammonia from their guano, which also covers the cave’s rocky floor. One false step can lead to a fall into a stream underneath. Another could land the scientists on one of the African rock pythons or forest cobras that slither along the ground.

Towner and CDC colleague Brian Amman, 54, discovered a decade ago that this Egyptian fruit bat is a natural reservoir for Marburg. That means the virus can live and grow inside the bats without harming the animal, and be excreted in its urine, feces or saliva.

By comparison, more than 40 years and over two dozen outbreaks after Ebola emerged in Central Africa, researchers still don’t know what animal or animals carry it, much less how it spreads to people.

The bat team includes CDC scientist Jennifer McQuiston and Luke Nyakarahuka, an epidemiologist at the Uganda Virus Research Institute, a longtime CDC partner. The CDC allowed a Washington Post reporter and photographer to accompany the team.

In a clearing of the Maramagambo Forest, the scientists’ workstation is a table under a tent. Curious baboons perch on nearby tree stumps. Black-and-white colobus monkeys peer from overhead branches. On a drive through the park in search of other bat roosts, the scientists’ Toyota Land Cruiser yields the right of way to a majestic waterbuck, its long, curved horns glinting in the sun.

Their task is to catch and glue trackers on 10 bats the first day, repeat the next day.

Only males are caught; the scientists don’t want to burden female bats that might be carrying pups. The scientists attach these small GPS devices to the backs of 20 bats to follow their movements. Towner releases a bat with a GPS device attached to it. Scientists are trying to determine flight patterns and how the bats transmit the Marburg virus to humans.

LEFT: Only males are caught; the scientists don’t want to burden female bats that might be carrying pups. CENTER: The scientists attach these small GPS devices to the backs of 20 bats to follow their movements. RIGHT: Towner releases a bat with a GPS device attached to it. Scientists are trying to determine flight patterns and how the bats transmit the Marburg virus to humans.

The tracking software has already prompted a stream of curses from Amman. To test it again, McQuiston and Nyakarahuka each cup a tiny unit in their palms and jog around the clearing to simulate bats on the move. That’s supposed to trigger readings. But the screen on Amman’s clunky CDC-issued laptop remains blank.

“If it comes back and says zero I don’t know what we’re going to do,” Amman mutters. Several minutes pass in silence. Then, ever so slowly, data points start to show up.

Now they can head to the cave.

Towner and Amman suit up in special helmets and face shields connected to battery-powered respirators that muffle their voices. Underneath protective gowns they wear Kevlar-lined waist-high pants to guard against snake bites. On their hands are cut-resistant leather gloves, like those worn by law enforcement, over two pairs of medical gloves to protect against bat bites. Towner also had a video mounted on his helmet.

“Hey, Brian, right down there is a bunch of males. You see ’em?” shouts Towner in the cave, his voice muffled by the head gear.

The bats take flight, which is when they relieve themselves of what Amman calls the “rain of pee and poo.”

Amman, holding a net, heads to where Towner is pointing, veering far away from an enormous python. He returns with two bats, which go into a pillowcase Towner is holding. Each pillowcase will hold about five bats since bats don’t like to be alone.

Only males are caught; the scientists don’t want to burden females carrying pups.

Catching the bats is the easy part. Getting the trackers on is trickier.

Towner searches for bats in an abandoned building in Queen Elizabeth National Park.

Amman dreads the prospect of using sutures because they are messy and complicated. But given the failure in Atlanta, he has brought the necessary kit. He hopes a last-minute purchase of different veterinary glue may do the trick.

Towner lifts the first bat out of a pillowcase, cradling it in his gloved hands. The bat is calm. Its big brown eyes blink, unaccustomed to the light. Towner places it stomach down, wings tucked in. One hand covers its head, the other its feet.

“Hello, big fella,” Amman says to the bat. “You have been selected to take part in the GPS Price is Right sweepstakes.”

He squeezes a thin line of glue on its back, another line of glue on the tracker and presses down gently but firmly.

For several long seconds, no one utters a word.

Towner breaks the silence. “That’s on pretty good,” he says softly, flicking at the device.

“WOO-HOO!” Amman shouts, relief washing over his face. Up go his arms, signaling a touchdown.

“I’m so happy, Jon,” he gushes. “I didn’t want to suture them. . . . I gotta tell ya. This is just better than Christmas.”

They glue the remaining units and release the bats. One near miss occurs when two bats chew through a bag and almost escape.

“Okay, dude,” Towner says to the last bat. He uncups his hands to free it. With a flap of its wings, the bat arcs slowly around the trees and glides into the forest.

Now the scientists wait, unsure whether the batteries will last, whether the signal will be able to break through cloud cover to reach the satellite, whether technology will be able to capture this crucial flight of nature.

Amman calls after the bat: “Bring us back some data!”

A herd of elephants makes its way through Queen Elizabeth National Park, which has great wildlife diversity, including fruit bats being studied by the CDC. Curious baboons perch on tree stumps at the national park and watch the scientists work. Bats infected with Marburg shed the virus in saliva, urine and feces, such as the guano seen above.

TOP: A herd of elephants makes its way through Queen Elizabeth National Park, which has great wildlife diversity, including fruit bats being studied by the CDC. ABOVE LEFT: Curious baboons perch on tree stumps at the national park and watch the scientists work. ABOVE RIGHT: Bats infected with Marburg shed the virus in saliva, urine and feces, such as the guano seen above.

First known case in United States

Barnes doesn’t remember touching any rocks or boulders in the cave. She and her husband were on a two-week safari. The bat cave in Queen Elizabeth National Park was something a tour guide suggested to fill an afternoon, before a trip to see Uganda’s famed mountain gorillas.

Barnes, her husband and seven others went inside on Christmas Day 2007. Barnes, then 44, was in the cave for about 15 to 20 minutes, in shorts and sandals. She remembers seeing the dark outlines of pythons and bats overhead.

“They were flying in and out and screeching and making all sorts of noise,” she said. “And the smell was super powerful. Everybody had their hand over their noses.”

A week later, she and her husband flew through London and then Iowa on their way home to Golden, Colo.

Her symptoms started en route — headache, rash, nausea — and worsened when she got home. Initially, doctors gave her painkillers and nausea medicine. As her liver, kidney, lungs, gallbladder and pancreas functions slowed, she was rushed to the hospital.

After 11 days, she was sent home. Despite tests for multiple diseases, she had no idea what had made her so sick. For months, she suffered abdominal pain, exhaustion and what she calls “mental fog.”

Astrid Joosten spent about 10 minutes in the cave six months after Barnes visited. Thousands of bats were flying around. “All were looking at us with very curious eyes,” recalled her husband, Jaap Taal.

She developed a high fever after she flew home to the Netherlands. She got worse, and hemorrhaged. Three days after she was put in an induced coma, Joosten died, with a confirmed Marburg diagnosis.

In Colorado, Barnes read about the case and asked her infectious disease doctor to rerun tests for Marburg. This time, the results were positive, the first known case in the United States.

Barnes now has another distinction. Not only is she the sole American survivor of Marburg, but her immune system is helping to develop a vaccine.

“She has awesome antibodies,” said James Crowe, a Vanderbilt University immunologist who is among the researchers who isolated one particularly powerful antibody from Barnes. An experimental vaccine is now in development.

For now, the most effective way to battle outbreaks like Marburg is stopping them at their source.

Monday Richard, with his daughter Lucy, recounts losing his pregnant wife and toddler son to Marburg outside his home in Uganda. Richard survived a 2012 outbreak that sickened 15 people in several districts and killed four. Red Cross workers take the temperature of people crossing the border from Congo into Uganda. Congo is battling the second largest Ebola outbreak in history. Pedestrians dip their shoes in a disinfecting solution as they cross the border from Congo into Uganda. Thousands cross on market day.

TOP: Monday Richard, with his daughter Lucy, recounts losing his pregnant wife and toddler son to Marburg outside his home in Uganda. Richard survived a 2012 outbreak that sickened 15 people in several districts and killed four. ABOVE LEFT: Red Cross workers take the temperature of people crossing the border from Congo into Uganda. Congo is battling the second largest Ebola outbreak in history. ABOVE RIGHT: Pedestrians dip their shoes in a disinfecting solution as they cross the border from Congo into Uganda. Thousands cross on market day.

‘Marburg has made me suffer’

The bat cave had always been a popular tourist attraction. “We used to have tourists walk down there,” said park veterinarian Margaret Driciru. “Health-wise, it really was not the right idea.” Most likely, other Marburg cases have gone undetected because the disease symptoms are similar to malaria and typhoid fever, common illnesses across much of Africa.

Now the park has posted warning signs that bats can carry the Marburg virus. Visitors must stay in an enclosed observatory with glass windows about 65 yards away.

The Uganda Wildlife Authority, which runs the country’s parks and is helping the CDC, also has an enormous stake in the project’s success. Wildlife is the top tourist attraction, and tourism is Uganda’s biggest source of revenue. But if wildlife carry diseases that kill humans and nonhuman primates, like the park’s famed chimps, tourists will stop coming.

In a meeting with a park warden, Towner and Amman explain how the trackers may show bats traveling to nearby towns in search of fruit. Any fruit the bat bites can be smeared with Marburg; a person, monkey or other animal eating that fruit can get infected.

“So they move up into the community?” warden Robert Mbagaya asks.

Yes, Towner says. He shows him a video on his cellphone of the bats with their GPS units.

“You see them speeding somewhere and you don’t know how far and for how long they go,” Towner explains. “But now we will know.”

After outbreaks of Nipah virus killed scores of people in Bangladesh over the past decade or so, scientists discovered that humans were getting infected from drinking virus-infected date palm sap. The sap is collected overnight from the trees. Bats were flying to the containers to drink the sweet sap draining from trees into collection pots. In the process, they were contaminating it with their saliva and excretions that are known to contain the virus.

Photographic evidence of bats urinating in the containers helped persuade villagers about Nipah’s dangers. Scientists hope flight patterns of the Marburg-carrying bats could be similarly persuasive.

“We wouldn’t be able to convince them to not sell the fruit, but we could make the argument to wash it first,” Towner said.

During last fall’s Marburg outbreak that sickened a cattle farmer’s family, “a traditional healer told the family [the sickness] was because of family conflicts,” Nyakarahuka said. Ugandan officials eventually met with nearly 150 community religious leaders and elders as part of outreach and education about the disease.

At the Kitaka gold mine, about 30 miles northeast of the bat cave, some residents also doubt that bats can cause serious disease despite a Marburg outbreak in 2007 that infected four miners, killing one. It was there that Towner and Amman discovered that the bats carried live Marburg virus. The mine was closed but has since reopened.

John Niwagaba, 53, searching for bits of gold outside the cave, said sick miners were bewitched. His proof: “I’ve been bitten by a bat but I didn’t get sick,” he said, during a break from pounding rocks.

At a nearby village, Monday Richard is the village’s sole survivor of a 2012 Marburg outbreak that sickened 14 others in several districts and killed four, including his pregnant wife and toddler. Villagers nicknamed him “Marburg.” His older children were not allowed to go to school for two months. He was no longer able to work as a motorbike taxi driver. Now he barely gets by as a banana farmer.

“Marburg has made me suffer,” he said.

Investigators can’t pinpoint how the outbreak began. But they strongly suspect the spillover was linked to bats in the Kitaka mine.

Richard’s seven-months-pregnant wife died while being transferred from one hospital to another. “She started bleeding from the nose. Then blood came out of her oxygen mask. It was all over her face,” he said.

He had to bury her and their 18-month-old son the same evening. There was no time for a funeral. On the side of the house, under a banana tree, their unmarked graves lie under a patch of red dirt.

CDC scientist Brian Amman, center, discusses tracking software with colleague Jonathan Towner, left, as Luke Nyakarahuka, an epidemiologist at the Uganda Virus Research Institute, right, and Apollo David Bogere observe.

A good sign

It’s the CDC team’s last day in the forest. Time for the final test.

A female vervet monkey and her baby look down from a tree as Amman and Towner huddle over a laptop. The lodge has no Internet service. The only connection comes from a portable device that seems to take forever to connect. Finally, a green bar appears on the screen. A good sign. Then it grows longer, an even better sign that data is loading.

Then the numbers pop up. Amman can see that Bat No. 14 logged more than 3,000 data points in one night. That means he flew a good distance. So did Bat No. 11.

“Oh man, this is awesome! This dude moved!” Amman exclaims, grinning widely.

Days later, Amman would be able to tell that the bats flew in different directions, some traveling up to 15 miles. At least two flew southwest toward a spot near another town on the other side of the forest. That could mean a broader area of infection risk.

Perhaps the bats were headed to a big fruit tree. Or another cave. Travel between far-flung roosts could be one way the virus is transmitted to other colonies. GPS coordinates show the two bats flew to an area two miles due east from an existing road. That is definitely something to be investigated, perhaps on a future trip.

For now though, the pair of scientists are quietly thrilled that their effort, years in the making, had worked.

“Good job,” Amman said quietly, giving Towner a high-five.

“Good job,” Towner replied.