When chimps first started to drop dead in front of him, Fabian Leendertz had no idea why. “It’s normal for wildlife to hide their weakness until they can’t anymore,” he says. One minute he’d be running through the Taï forest in the Ivory Coast, tracking chimps who looked in the peak of health. The next minute, the animals would be lying down and breathing heavily. A few hours later, they were dead.

Initially, Leendertz thought that the chimps had been floored by a virus like Ebola or measles. But after examining the bodies, he and his team identified a culprit that wasn’t even on his list of suspects: anthrax.

Anthrax is typically the work of a bacterium called Bacillus anthracis, which produces a cocktail of powerful and lethal toxins. But the microbe that attacked the chimps was a new strain of Bacillus cereus, which had acquired many of the same toxin genes. Very little is known about this strain of rainforest anthrax, known as Bcbva. But Leendertz and his colleagues from the Robert Koch Institute in Berlin have now shown that it’s a startlingly prolific killer, responsible for more than a third of mammalian deaths in the Taï forest. And it is likely to wipe out the local chimpanzees within the next 150 years.

Scientists have been working in the Taï forest for 28 years, collecting the bones of dead mammals and performing necropsies on any carcasses. Leendertz began to see the chimps die in 2001. By analyzing this archive, his team confirmed that Bcbva was responsible for around 38 percent of local wildlife deaths in the forest. The strain may be a recent discovery, but it’s clearly been busy.

Genetically, the new strain is incredibly diverse. Two strains of Bcbva, which were captured just six kilometers and two years apart, had 69 genetic differences between them. That’s around three times as many as you’d expect to see in B. anthracis—the bacterium that typically causes anthrax—across an entire mountain range over a dozen years. This diversity suggests that Bcbva isn’t a newcomer. It has probably been circulating in sub-Saharan Africa for a very long time. But if that’s the case, why did it take until 2001 to spot chimps dying from this strain?

“It definitely challenges our understanding of how anthrax works,” says Katie Hampson from the University of Glasgow, who has studied the disease in Africa before. “We know almost nothing about anthrax in rainforest settings.” Classically, it’s a disease of the dry savannah. When it rains, anthrax flares up in largely predictable outbreaks, wipes out a few hundred antelope, and then fades away. But in Taï, it seems to be active throughout the year, and it peaks during the dry season, not the wet one.

In the savannah, anthrax almost always infects hoofed grazing mammals, which ingest soil laced with bacterial spores. Although it can spill over into humans, until 2001, there was no record of it afflicting wild primates. Now, we know that the Taï strain hits chimpanzees, as well as other unusual hosts like mongooses and porcupines. It even affects monkeys that spend all their time in the treetops, far away from contaminated soil. “We don’t know how they get infected,” says Leendertz. “How do the spores make it up in the trees?”

Carrion flies may be involved. These insects land on fresh carcasses to feed and lay their eggs. In the process, they take up some of the DNA from those bodies, as well as the local microbes. By capturing these flies, and sequencing the DNA within them, Leendertz’s team is able to take an unorthodox census of the mammals and microbes in an otherwise inaccessible jungle. And they’ve found that between 5 and 12 percent of these flies are infected with the Bcbva strain. Perhaps they shuttle the bacterium between the forest floor and the canopy, contaminating fruit that the monkeys—and chimps—eventually eat.

Anthrax doesn’t pass between animals, so infected individuals have to pick up the spores from some other source. As Leendertz says, “If I’m sick, you won’t get it from me unless you eat me when I’m dead.” But the people of the Taï forest do eat a lot of bushmeat. Could they pick up anthrax infections in the process? No one knows. “In these rural areas, no one really knows what people die of. In reports, it always says things like malaria or cholera, but these are not properly diagnosed,” says Leendertz. “We have stories of people finding dead animals in the forest, eating them, and dying. But we can’t link those to anthrax yet.” He and his team are now working with local hospitals to test for the bacteria in the blood of patients.

“This study is all the more pertinent because the western subspecies of chimpanzees [the one that lives in Taï] is now listed as critically endangered,” says Tatyana Humle, from the University of Kent. This isn’t the only example where disease epidemics are threatening great apes. In the Congo basin, Ebola has killed thousands of gorillas. And in Taï, anthrax is a serious threat.

Based on the birth and death rates of chimps, and how often they reproduce, the team calculated that there’s an 89 percent chance that anthrax will eliminate the entire Taï population within 150 years—and that’s if nothing else goes wrong. If poaching increases, or other diseases enter the fray, the sand will drain from the hourglass even faster. “These calculations are very conservative,” says Leendertz. “It looks bad for them. We’re trying to protect them and we did vaccinations but are still testing if they worked.”

The only spot of good news is that Bcbva doesn’t seem to be active in other parts of Africa. “We can’t say that a similar special situation doesn’t exist somewhere else,” says Leendertz, “but it doesn’t seem like a general problem for chimps. There’s something special about Taï.”