Borrelia is a genus of bacteria of the spirochete phylum. It causes borreliosis, a zoonotic, vector-borne disease transmitted primarily by ticks and by lice, depending on the species. The genus is named after the French biologist Amédée Borrel (1867–1936). There are 52 known species of Borrelia. 21 are members of the Lyme disease group, 29 belong to the relapsing fever group, and two are members of a third group.