An outbreak of the deadly Ebola virus in Uganda has alarmed scientists. While no cases have yet been discovered outside Africa, the U.S. has started screening all arrivals from Uganda. Ebola is among the deadliest of pathogens capable of jumping from wild animals to humans—just as COVID-19 likely did. It’s called spillover. Disease detectives warn the threat of spillover has never been higher as urban populations grow and come into contact with wild animals and their viruses. Since 2009, American scientists have discovered more than 900 new viruses. Now, the U.S. government is doubling down, sending virus hunters to global hotspots to find the next deadly virus before it finds us. We joined a team from the University of California Davis and their Ugandan partners in the rugged Impenetrable Forest on the search for Pathogen X.
We landed in Kihihi, a speck of a town in southwest Uganda. As we headed off to the Impenetrable Forest, we soon saw how it got its name. It’s so thick with trees, vines, and roots that Ugandans call it the place of darkness. As our 4x4s bumped and swerved along deeply rutted tracks, we passed tea farmers, loggers, villagers, all living on the edge of the forest, where the risk of infectious disease spilling over from animals is highest. Wildlife epidemiologist Christine Johnson handicapped the stakes.
Bill Whitaker: How would you rate the odds of another pandemic?
Christine Johnson: I would say another pandemic is guaranteed.
Bill Whitaker: Guaranteed.
Christine Johnson: It’s not a matter of if, but when. That’s why we’re so committed to preparation.
Johnson leads the UC Davis team and has been hunting viruses around the globe for decades. We were headed to an abandoned mineshaft to look for bats. Johnson told us bats are prime suspects for spillover. They harbor more viruses lethal to humans than any other mammal. New bat species—and new viruses—are still being discovered.
Bill Whitaker: It seems like a really daunting task for you to find Pathogen X before it finds us?
Christine Johnson: It’s definitely achievable.
Bill Whitaker: It is achievable.
Christine Johnson: Absolutely. It’s all here right now, right? It’s not like we’re exploring outer space. All of these viruses, and – and – and all of the wildlife are right here on our planet.
The bats would start flying at dusk. We waited as the UC Davis team and their Ugandan partners hung a fine mesh net across the entrance of the cave. We wore masks and goggles to protect ourselves against any early risers.
Benard Ssebide, one of Uganda’s top wildlife vets, told us this area used to be all forest. Now, villagers had planted a cornfield right up to the mouth of the bat cave, increasing the risk of spillover. As if on cue, we watched women carrying water cut through the cornfield, while school children ran home.
Bill Whitaker: The transfer between bats and humans, it’s much more likely when you’ve got people living so close.
Benard Ssebide: Exactly. The population has grown. People have moved into areas they’ve never aspired before. That shrinkage of the buffer, the habitat, between the people and wildlife has become so narrow, so that increases the contact.
Bill Whitaker: We were talking about people who are now living right on the edge of the Impenetrable Forest.
Benard Ssebide: Exactly. Governments cannot stop people from moving into some of these areas because they have nowhere else to go.
Bats are known to carry coronaviruses—the same virus family that spawned COVID-19—as well as lethal ebola viruses.
Christine Johnson Make sure there’s boots.
So we had to dress head to toe in protective gear. Once the hazmat suit was on, we added two sets of gloves, a mask, and a face shield to guard against flying guano and other toxins.
Bill Whitaker: Once we begin, I must assume everything is contaminated?
Christine Johnson: Exactly
The Impenetrable Forest was soon pitch black and we had only the light from our headlamps to guide us. Soon, they’d trapped a large Egyptian fruit bat. Wildlife vet Benard Ssebide gently disentangled it and put it in a fabric sack. We followed him back to the makeshift lab, glowing in the dark.
The bat sacks quivered in the ghostly light. It felt like we were on the set of a sci-fi movie.
Bill Whitaker: Oh he’s a big guy.
Up close, the bats did little to dispel their fearsome reputations. We watched as the fruit bat grew agitated, trying to escape. The scientists held its nose to a test tube filled with a mild anesthetic. Finally, the bat succumbed. Epidemiologist Christine Johnson told us the bat would be swabbed for a suite of viruses.
Bill Whitaker: Does this hurt the bat at all?
Christine Johnson: No, it doesn’t hurt the bat. We—we get the right size swab so that we’re just doing an oral sample. It might be a little uncomfortable.
The bat’s wings were examined for parasites and ticks that might also have pathogens. All the samples would be sent to a lab for DNA sequencing. Johnson told us a virus’s genetic code can help identify which might cross to humans.
After the tests were done, the bats were released, groggy but unharmed.
The next day we joined Tierra Smiley Evans, a UC Davis epidemiologist and wildlife vet. We were looking for monkeys and baboons. Like bats, primates carry many viruses that have leapt to people. Smiley Evans told us catching an outbreak early, at the point of spillover, is vital to containing it.
Bill Whitaker: It sounds like there’s no shortage of viruses that can infect humans that come out of the forest?
Tierra Smiley Evans: There are probably more pathogens that we don’t know about than ones that we do know about. We need to gather more information and more intelligence about what may be out there and able to spill over before it does.
Bill Whitaker: So they come right down to the hospital?
Tierra Smiley Evans: Yeah in the back, it butts up right against the forest.
We met her at the Bwindi Community Hospital on the edge of the forest.
Bill Whitaker: This really is something.
It’s so close, we saw baboons casually strolling on the hospital grounds, sometimes getting into patients’ rooms.
Tierra Smiley Evans: Whenever you’re creating a new opportunity for humans to come in contact with wildlife populations that they were never in contact with before, you’re creating a brand-new situation.
Bill Whitaker: So as human populations grow, that’s pushing us into areas we’ve never been before.
Tierra Smiley Evans: Exactly.
Bill Whitaker: Putting us into contact with animals we’ve never been in contact with before.
Tierra Smiley Evans: Exactly.
To find out what viruses the baboons were carrying, Smiley Evans pioneered a simple but groundbreaking method to collect saliva samples: the stealth banana. Tied to a string, the banana is tossed to the curious baboons. But hidden inside is an oral swab coated in something sweet that the baboons love to chew. Smiley Evans and Ugandan wildlife vet Bukamba Nelson had prepared the bananas earlier in the day.
Tierra Smiley Evans: So we have tried strawberry jam. We have tried mango juice.
Bill Whitaker: Have you found they like one more than the other?
Tierra Smiley Evans: The difference is that sometimes they’ll chew on that swab for longer periods of time with a different attractant versus another. And that’s what we really want.
It’s like bubble gum for primates. When the sweet is gone the baboons throw the swab away, leaving behind plenty of saliva that can be decoded for viruses.
But family politics can sometimes get in the way. Meet the big daddy of this troop. He wasn’t about to let anyone else get even a mouthful. Mom hauled the babies out of the way until, finally, the coast was clear. By then, all that was left were soggy leftovers. Wildlife vet Bukamba Nelson told us it was worth the wait. It was rare to see babies venture this close.
Bill Whitaker: So you got saliva samples from the babies?
Bukamba Nelson: Yeah, yeah.
Bill Whitaker: That’s unusual?
Bukamba Nelson: It’s very unusual.
Bill Whitaker: So what do you get from the babies that you don’t get from the adults?
Bukamba Nelson: You never know. I might find a particular disease in this age bracket, which might not be found in the juvenile or the females. Sex, age, all that plays a lot in disease intelligence.
Disease intelligence that also includes training villagers to be on the look-out for any unusual fevers or flu-like symptoms. Scientists can then match human illnesses to the animal viruses they’ve found in the same area. Smiley Evans told us it was putting pieces of a puzzle together.
Tierra Smiley Evans: All the samples are tested in the same way for the same pathogens. So, the goal is that if we’re sampling at the same time, in the same area we can start to connect the dots and understand when there’s been transmission of a particular virus.
One of the most closely monitored species in the Impenetrable Forest are its star residents, the endangered mountain gorilla. Nearly half the world’s remaining gorillas are here, 459 at last count. They’re always on the move, so we set off to find them. One ridge led to another, each steeper than the last. The forest was so dense there was no sunlight. And no gorillas. Wildlife vet Benard Ssebide assured us we were on the right path.
Bill Whitaker: Are you seeing signs of the gorillas around here?
Benard Ssebide: Yeah, I’ve seen some already.
Our porters breezed along unfazed. We? Not so much.
Then, hours after trekking, suddenly, there they were. We spotted a mother first, high in the trees gorging on twigs. Soon we were surrounded by all 19 members of an extended family, including a massive silverback and another mother cradling her infant. We had put on our masks, not to protect ourselves, but to protect the gorillas from any infection we might be carrying. Amy Bond is with Gorilla Doctors, an international conservation group. She told us how they identify each gorilla.
Amy Bond: Just like humans where we each have our own unique fingerprint that helps us be identifiable as an individual, gorillas have unique nose prints.
Bill Whitaker: A nose print?
Amy Bond: A nose print and that’s what allows us to identify those individuals. And so we go through and we make sure we get each individual in the group that we can do a visual assessment, looking for signs of illness or injury.
Bond and wildlife vet Benard Ssebide told us that gorillas are susceptible to many of the same pathogens that we are and they can be an early harbinger of disease. The gorillas are monitored daily for any warning signs.
Amy Bond: When they’re sick, it’s very similar, right?
Benard Ssebide: Yeah
Amy Bond: Runny nose, coughing, sneezing
Benard Ssebide: Coughing, sneezing
Amy Bond: They’re not moving, they don’t want to eat.
If a gorilla is lying down, Ssebide told us, they’ll assess if he’s resting or if something else is preventing him from moving. We spotted one young male on his own, but Amy Bond told us he was likely suffering from a problem of a different sort.
Amy Bond: You can also sometimes tell which silverback is dominant by the number of females around him.
Bill Whitaker: So this poor guy sitting over here? (chuckles) He’s just out.
Amy Bond: He’s just always second choice.
Aside from a case of wounded male pride, Bond told us this family appeared to be thriving. But their future isn’t guaranteed. And if theirs isn’t, neither is ours. Bond told us as spillover threats grow, it’s impossible to separate human health from the health of the natural world. As UC Davis scientists continue their work, the search for Pathogen X is a search for what threatens the animals of the impenetrable forest as much as it threatens us.
Produced by Heather Abbott. Associate producer, LaCrai Mitchell. Broadcast associate, Natalie Breitkopf. Edited by Peter M. Berman.