Borrelia is a genus of bacteria of the spirochete phylum. It causes Lyme disease, also called Lyme borreliosis, a zoonotic, vector-borne disease transmitted primarily by ticks and by lice, depending on the species of bacteria. The genus is named after French biologist Amédée Borrel (1867–1936), who first documented the distinction between a species of Borrelia, B. anserina, and the other known type of spirochete at the time, Treponema pallidum. This bacterium must be viewed using dark-field microscopy, which make the cells appear white against a dark background. Borrelia species are grown in Barbour-Stoenner-Kelly medium. Of 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 genetically distinct third gro