In late June 2009, a small group of mountain gorillas in Rwanda’s Volcanoes National Park began to fall ill. One by one, 11 of the dozen apes started exhibiting severe respiratory problems. Within days of the park researchers’ noticing the outbreak, one adult female and her 4-year-old son went missing from the group. When wildlife veterinarian Jean-Felix Kinani and his colleagues went looking for the pair, they found the mother lying dead, face down on the ground. Her 4-year-old son sat beside her, "vocalizing, calling other gorillas,” Kinani recalls. Kinani helped treat the young male gorilla with the antibiotic ceftriaxone, returned him to the group, and brought the mother’s body back to the lab for necropsy.

Kinani found stones in the dead gorilla’s stomach, which indicated "she was very weak, trying to eat what was close,” he says. Her lungs showed clear signs of pneumonia, and he found several parasites in her gastrointestinal tract. But without more advanced microbiology equipment, he was unable to identify the exact cause of death. "It’s very rare to have pure bacterial pneumonia without some sort of immune-disturbing event,” like a virus, says Mike Cranfield, a University of California, Davis, wildlife veterinarian and executive director of the Mountain Gorilla Veterinary Project (MGVP).

As more gorillas began to fall victim to the mysterious disease, the veterinarians on site delivered antimicrobial therapy, and the group suffered just one more casualty—a 3-day-old infant. Kinani and his colleagues shipped tissue samples from the two deceased animals to collaborators at Columbia University’s Center for Infection and Immunity to have a closer look. "We are known to have tools to find the needle in the haystack,” says Center experimental pathologist Gustavo Palacios.

Palacios and his colleagues used a method called MassTag PCR, which allows researchers to detect 30 different pathogens simultaneously. Running the samples against a variety of panels containing agents that could be responsible for the respiratory symptoms exhibited by the animals, the researchers found signals for human metapneumovirus (hMPV)—a common cause of lower respiratory infection in children.

The finding didn’t come as a complete surprise. While biologists have previously tracked the transmission of disease from animals to humans, wildlife researchers have long suspected the wild apes were falling victim to human diseases, as sometimes happens in zoos. Indeed, a 2008 molecular analysis identified hMPV as the cause of a pneumonia outbreak in wild chimpanzees in 2006. And with ecotourism on the rise, more and more foreign visitors travel to see wild animals up close and personal, increasing the risk of disease transmission. "I would say it’s probably been happening for quite a while,” Cranfield says. "It was just a matter of finding it.”

Sequencing the virus confirmed its identity, and a phylogenetic analysis suggested that the likely origin of the virus was a recent outbreak of hMPV in South Africa. "There was transmission between humans and gorillas within months, probably weeks, maybe even days,” Cranfield says.

Cranfield hasn’t pinpointed where or when humans infected the Rwandan gorillas, but the broader goal is to continue encouraging ecotourism—which has actually helped increase gorilla populations by 26 percent in the past 7 years—while safeguarding the animals from human disease. Simple precautions, such as wearing surgical masks or increasing the minimum distance between visitors and wildlife, can help curb transmission.

"I think the benefit of tourism and research is still bigger than the harm we do,” says wildlife epidemiologist Fabian Leendertz of the Robert Koch-Institut in Germany, who participated in a study that identified another common human respiratory virus in wild chimpanzees. "That’s what saves [wild great apes] from being killed or poached,” he explains. "It’s important to keep the balance.”

This balance is even more important given the uncertainty about how wild apes will respond to human viruses, says wildlife veterinarian William Karesh, executive vice president for health and policy at EcoHealth Alliance.

In addition, it’s important to study what aspects of a virus allow it to jump between species, adds Charles Chiu, director of the University of California, San Francisco’s Abbott Viral Diagnostics and Discovery Center. "Unless we understand the threat, we’re not going to be able to address it.”

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