The Borrelia genus contains about 36 species of spirochete bacteria. Among them, a group of several closely related species (genospecies) named Borrelia burgdorferi sensu lato was isolated. At least twelve of them are pathogenic and cause Lyme disease (Lyme borreliosis). The most important are: Borrelia burgdorferi sensu stricto, Borrelia garinii and Borrelia afzelii. Lyme disease is transmitted to humans and other mammalian species by Ixodes ticks. Humans are infected by contact with saliva or vomit of the tick carrying spirochetes. The first stage of Borrelia burgdorferi infection is the appearance of expanding area of redness called erythema migrans, which appeares at the site of the bite. In the second stage of infection, bacteria get into organs such as the heart, the motion organs, the nervous system. In the last stage of infection there is irreversible damage to infected organs. The main reservoir of pathogenic bacteria Borrelia burgdorferi are rodents.