The endangered mountain gorillas of Central Africa already face poaching, agricultural encroachment, and deforestation. Now there’s another threat to add to the list: two potentially fatal viruses that could be passed from humans to gorillas and other primates (including chimpanzees and orangutans) as easily as the common cold.

HRSV (human respiratory syncytial virus) and HMPV (human metapneumovirus) were recently found to be the cause of death of chimpanzees in the western African nation of Ivory Coast. Scientists are concerned that eco-tourists visiting chimpanzees, orangutans and mountain gorillas in their natural habitats could unwittingly introduce the two viruses into these endangered populations and spark a devastating outbreak.

Currently, tourists pay hundreds of dollars each for a permit to track gorilla groups into the mountains and spend an hour in their presence. This money is a vital source of income for the governments of Rwanda, Uganda and the DR Congo, which would otherwise be hard-pressed to find funding to protect and maintain the national parks the gorillas call home. Mountain gorillas are a critically endangered species, with fewer than 700 individuals left in existence.

Tourists who have cold- or flu-like symptoms are told not to participate in the trek, and while this approach is a good start to protecting the health of gorillas, it is flawed in several ways. First, tourists have spent a great deal of money on the permits, which are non-refundable, and flights to and from Africa are very expensive particularly for short-term travelers. This means that tourists may be tempted to hide their symptoms so they get the most value for their money.

Second, HRSV and HMPV do not always demonstrate symptoms in humans, so people are not likely to know they are carriers at all.

In order to avoid transference of HRSV or HMPV from humans to apes, scientists are proposing stricter safety measures such as requiring tourists and researchers to “produce proof of vaccinations and to disinfect all clothing and footwear before being allowed near the animals.” One of the simplest and most effective methods of lowering the risk of infection is for people to wear masks, such as those recommended against avian flu. But this poses its own set of problems.

As Dr Fabian Leendertz, a wildlife epidemiologist at Berlin’s Robert Koch-Institute, pointed out to The Guardian: “If you have spent all that money, the very least you want is a photograph of yourself with the gorillas. And the photograph doesn’t look as good if you have to wear a mask. But we hope the type of people who go on these holidays will take that responsibility.”