Batrachochytrium dendrobatidis (Bd) is a chytrid fungus that causes chytridiomycosis in amphibians. Only named in 1999, Bd is a proximate driver of declines in global amphibian biodiversity. The pathogen infects over 350 species of amphibians and is found on all continents except Antarctica. However, the processes that have led to the global distribution of Bd and the occurrence of chytridiomycosis remain unclear. This review explores the molecular, epidemiological, and ecological evidence that Bd evolved from an endemic ancestral lineage to achieve global prominence via anthropogenically mediated spread. We then consider the major host and pathogen factors that have led to the occurrence of chytridiomycosis in amphibian species, populations, and communities.