Lyme disease, caused by the spirochete Borrelia burgdorferi, is the most common vector-borne illness in the United States. B. burgdorferi sensu stricto causes disease in North America; B. garinii and B. afzelii are more common in Europe. Ixodes ticks transmit the disease: I. scapularis, which also transmits babesiosis and anaplasmosis, is found in the northeastern and midwestern United States; I. pacificus is found in the western United States. The white-footed mouse is the preferred host for larval and nymphal ticks. Adult ticks prefer the white-tailed deer as host. The tick must feed for 24 h to transmit the disease.