Chytrid fungi are ubiquitous and primitive. They can be found in deserts, as well as arctic and tropical regions. "They're very important in aquatic systems," Stephenson says. "If you toss debris into relatively clean water, you're going to attract chytrids. They're involved in the early stages of decomposition." Most chyrtids are harmless, he says, so scientists were surprised to discover that one, Batrachochytrium dendrobatidis, has been killing populations of frogs and other amphibians in recent years. The global die-off has gotten so dire that some scientists suggest it heralds the beginning of the world's sixth mass extinction event (pdf).

The fungus causes skin lesions that make it impossible for normal respiration to occur through the skin. In this image, the presence of spherical bodies in the skin cells of a northern leopard frog indicates that the frog is infected.