Borrelia burgdorferi is a bacterial species of the spirochete class of the genus Borrelia. B. burgdorferi exists in North America and Europe and is the only causative agent of Lyme disease in the United States. Borrelia species are considered diderm (double-membrane) bacteria rather than Gram-positive or negative. Lyme disease is a zoonotic, vector-borne disease transmitted by the Ixodes tick (also the vector for Babesia). The infected nymphal tick transmits B. burgdorferi via its saliva to the human during its blood meal.