Lyme disease, unknown in the United States two decades ago, is now the most common arthropod-borne disease in the country and has caused considerable morbidity in several suburban and rural areas. The emergence of this disease is in part the consequence of the reforestation of the northeastern United States and the rise in deer populations. Unfortunately, an accurate estimation of its importance to human and animal health has not been made because of difficulties in diagnosis and inadequate surveillance activities. Strategies for prevention of Lyme disease include vector control and vaccines.