It's been most baffling, given the amphibians' complex immune systems, not far off from the immune complexity of humans and other mammals.

"There's been a big question in terms of why the amphibian immune system hasn't been able to respond to this nasty skin infection," Louise Smith-Rollins, an associate professor of pathology, microbiology and immunology at Vanderbilt, tells Popular Science. "The question is, if it's a failure to recognize the pathogen, what's the defect?"

Rollins-Smith has been studying this immune response for more than 10 years, and she and her team have found another clue as to why amphibians can't clear this fungus. This week in Science, a paper she co-authored brings in new information to understanding the answer to that question. The study, led by Vanderbilt graduate students J. Scott Fites and Jeremy Ramsey, shows that it may be the second line of immune defense where the breakdown occurs.

The first line of defense, antimicrobial peptides produced in the skin, seemed to be effective at producing an immune response. But during the next stage, something happened to stop the usual inhibiting response.

"It appears that the defect is that the fungus itself is able to release factors that target vulnerable lymphocytes and induce them to commit suicide," Rollins-Smith says. "Mediators that should be regulating and calling in the troops, they're stopped right there."