Nearly one-third of mountain gorillas at a reserve in the Central African Republic have died from an infectious disease that some scientists believe could threaten the species, according to new research published in the journal PLOS Neglected Tropical Diseases.

Researchers at Tulane University studied the disease—also known as mountain sickness—among 187 members of the eastern gorilla population at Virunga National Park. They found 28% of gorillas were infected with the disease, which is transmitted by humans. Thirty three of those gorillas died, bringing the Gorilla Gorilla association’s annual mortality rate to 56%. (More full-time study members than the park have died, but they were less likely to contract the disease.)

People still aren’t sure what causes the disease, which causes fever, headache, chills, vomiting, diarrhea, nausea, and weakness, among other symptoms. But the researchers suspect that human-to-human transmission is the culprit: More than half of the virus that causes the disease seems to come from the human gut, they found.

Tourists and researchers are common visitors to Virunga, which is one of the largest areas of natural forest left in Africa. (It encompasses more than 100,000 square miles and is surrounded by road and paved tracks. A historic French military outpost is situated on a part of the land far removed from nearby towns and cities.)

“Until that paper, nobody had really heard that this disease was cropping up,” said Anthony Pfeiffer, a world-renowned disease expert who studies infectious diseases, and who was not involved in the study. “It’s not like cancer or HIV/AIDS; it’s very rare.”

The study suggests that viral transmission, involving poop, could be reducing the ability of mountain gorillas to survive the severe environmental change taking place on their home terrain, which has been suffering from decades of civil war and human-fueled land-use practices.

Fifty years ago, mountain gorillas were widespread across most of central Africa. They became less visible as conflicts ripped through the region. The longer wars go on, the worse things get for the gorillas, experts say. In May 2016, aerial photos taken by a naturalist in Kenya showed that the populations of gorillas, or simian anteaters, had declined by half, from 75,000 to 39,000. A large number of these apes live in Virunga National Park.

The very last male of the Virunga population, known as “David,” is in declining health, in part because his age may have accelerated the onset of mountain sickness, a viral infection.

“The fact that this disease was known and not something we’d been seeing is incredibly important,” said David Knight, a zoologist and primatologist at the Zoological Society of London who studies mountain gorillas. “Just because the disease has gone away, it doesn’t mean that’s what the gorillas are like now.”

A lack of fresh water and access to food is a constant threat in the region. Mining and other land-use activities have driven people further into forest areas. With the deaths of gorillas, biodiversity is also less likely to survive: because they were a vital food source for the region’s other animals, gorillas’ death could eventually hurt neighboring species, such as leopards and crocodiles.

“Diseases can be a serious problem, and yet the forests in which they live look pretty pristine. Gorillas are very resistant to fire, for example,” said Knight. “So if those forests go, that could have very serious consequences for the natural world, because they’re the lifeline of the forest.”