Lyme disease is caused by bacteria from the genus Borrelia. At least 11 Borrelia species have been described, 3 of which are Lyme related. The Borrelia species known to cause Lyme disease are collectively known as Borrelia burgdorferi and have been found to have greater strain diversity than previously estimated.
Three closely-related species of spirochetes are well-established as causing Lyme disease and are probably responsible for the large majority of cases: B. burgdorferi predominant in North America, but also in Europe), B. afzelii, and B. garinii (both predominant in Eurasia). Some studies have also proposed that B. bissettii and B. valaisiana may sometimes infect humans, but these species do not seem to be important causes of disease.