The global spread of the pathogenic fungus Batrachochytrium dendrobatidis (Bd), which causes chytridiomycosis, has led to the decline or extinction of up to 200 frog species. This disease is highly transmissible and virulent, and its emergence and spread are the most likely primary cause of "enigmatic" declines in frog populations. The disease has been linked to mass mortality events, which are often undetected due to the rapid decomposition of dead frogs. Experimental evidence shows that Bd causes fatal disease in healthy frogs, and its presence during declines is consistent with its role in mortality. Bd is highly conserved genetically and has spread globally, likely from southern Africa. The disease's impact is the most significant recorded loss of vertebrate biodiversity due to disease. Two hypotheses explain Bd's spread: the "spreading pathogen hypothesis," where Bd spreads into naive populations, and the "emerging endemic hypothesis," where environmental changes increase Bd's virulence. Evidence supports the spreading pathogen hypothesis, as Bd has been found in amphibian populations before their declines, and its genetic similarity suggests recent emergence. The disease's rapid spread and high virulence have led to the need for conservation strategies to prevent further declines. Global cooperation is essential to control Bd, as it poses a significant threat to amphibian populations worldwide.The global spread of the pathogenic fungus Batrachochytrium dendrobatidis (Bd), which causes chytridiomycosis, has led to the decline or extinction of up to 200 frog species. This disease is highly transmissible and virulent, and its emergence and spread are the most likely primary cause of "enigmatic" declines in frog populations. The disease has been linked to mass mortality events, which are often undetected due to the rapid decomposition of dead frogs. Experimental evidence shows that Bd causes fatal disease in healthy frogs, and its presence during declines is consistent with its role in mortality. Bd is highly conserved genetically and has spread globally, likely from southern Africa. The disease's impact is the most significant recorded loss of vertebrate biodiversity due to disease. Two hypotheses explain Bd's spread: the "spreading pathogen hypothesis," where Bd spreads into naive populations, and the "emerging endemic hypothesis," where environmental changes increase Bd's virulence. Evidence supports the spreading pathogen hypothesis, as Bd has been found in amphibian populations before their declines, and its genetic similarity suggests recent emergence. The disease's rapid spread and high virulence have led to the need for conservation strategies to prevent further declines. Global cooperation is essential to control Bd, as it poses a significant threat to amphibian populations worldwide.