Antarctic penguins, already challenged by climate change and loss of habitat, are now facing a new threat -- tourists, and the diseases they bring with them when visiting the frigid region.

Penguins, along with many other Antarctic species, are thought to have immune systems less capable of dealing with the world's common pathogens because of their long isolation from them at the bottom of the world.

Humans have been in the region for only the last 200 years or so, researchers point out.

However, a growing eco-tourism industry brought more than 37,000 visitors to the Antarctic in the last year, and as many as 4,000 scientific researchers can be on the continent at any one time.

That doesn't bode well for penguins, says Wray Grimaldi of the University of Otago in New Zealand.

"The effects of both a growing tourism industry and research presence will not be without consequences," she says. "Penguins are highly susceptible to infectious diseases."

As proof of that, she cites a survey she and her colleagues made of infectious diseases found in captive penguins.

Pathogens including avian flu, E. coli, salmonella and West Nile virus have all been found in penguins in captivity during the last 50 years, she and her fellow researchers report in a study published in the journal Polar Biology.

There have also been incidents of mass mortality in penguins in the Antarctic since 1969, they say, including two outbreaks of Avian pox that killed hundreds of Gentoo penguins in 2006 and 2008.

That and other infectious agents may have arrived in Antarctica with migrating seabirds, Grimaldi acknowledges, but it is also possible that some were introduced through human presence in the region.

There is insufficient evidence at present to confirm or rule out either possibility, she says.

Climate change may also lead to the emergence of more penguin diseases in Antarctica, she warns, a prediction other experts say they agree with.

"Climate change may result in a number of stressors that make it more difficult for penguin populations to deal with disease," says Claire Christian of the Antarctic and Southern Ocean Coalition. 

Rising temperatures could reduce food sources like krill the penguins depend on, which might leave them less capable of fighting off diseases, she says.

Vigilance will be necessary to protect the penguins, Grimaldi argues.

"A coordinated monitoring system needs to be in place," she says. "That way, responses can be directed by science."