Batrachochytrium dendrobatidis ( bə-TRAY-koh-KIT-ree-əm DEN-droh-bə-ty-dis), also known as Bd or the amphibian chytrid fungus, is a fungus that causes the disease chytridiomycosis in amphibians.

Since its discovery in 1998 by Lee Berger and species description in 1999 by Joyce E. Longcore, the disease devastated amphibian populations around the world, in a global decline towards multiple extinctions, part of the Holocene extinction. A recently described second species, B. salamandrivorans, also causes chytridiomycosis and death in salamanders.

The fungal pathogens that cause the disease chytridiomycosis are known to damage the skin of frogs, toads, and other amphibians, disrupting their balance of water and salt and eventually causing heart failure, Nature reports.

Some amphibian species appear to have an innate capacity to withstand chytridiomycosis infection due to symbiosis with Janthinobacterium lividum. Even within species that generally succumb, some populations survive, possibly demonstrating that these traits or alleles of species are being subjected to evolutionary selection.

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