Antarctica remained largely untouched until roughly 200 years ago, and now, more than 10,000 people travel there every year. But tourists bring more than cameras. Scientists are warning that pathogens brought by visitors could threaten the continent’s most iconic inhabitant: the penguin.

Isolation has left local wildlife populations particularly vulnerable to diseases commonplace elsewhere in the world. “The effects of both a growing tourism industry and research presence will not be without consequences,” Wray Grimaldi of the University of Otago in Dunedin, New Zealand, said to New Scientist. “Penguins are highly susceptible to infectious diseases.”

One group of researchers found 20 different fecal pathogens on just 15 pairs of tourist boots.

Her team of Antarctic researchers found multiple infectious agents—bacteria such as salmonella and E. coli, viruses such as West Nile and the Avian pox virus—in captive penguins dating back to 1947. Outbreaks from those diseases have killed thousands of penguins over the years, the team reported in a paper published this month in the journal Polar Biology.

Another theory is that migrating animals may have brought diseases to Antarctica, as the warming climate is attracting more species than ever before. But previous studies have identified tourist boots as vectors for disease transmission. One group of researchers tested 72 tourists' boots and found 20 different fecal pathogens on just 15 pairs of shoes.

Norman Ratcliffe, an Antarctic ecologist from the Antarctic Survey in Cambridge, United Kingdom, told New Scientist that the evidence blaming tourists for sick penguins is lacking. He said that tourism companies are very strict on what they let visitors bring on their journey. “The tour companies are quite careful to make sure everyone cleans their boots before they go ashore,” he said. “They don't allow any animal products to be taken ashore.”

This story originally appeared on The Atlantic.