Lyme borreliosis has a global distribution and is recognized as a complex, multistage, multisystem inflammatory infectious disease of both humans and animals. Lyme borreliosis is a zoonosis in which humans are incidental hosts that do not contribute to its maintenance in nature. In the eastern part of the United States, humans most often acquire Lyme borreliosis from infective nymphal ticks, usually in late spring and early summer, and infrequently from adult ticks, which feed most often in the fall and winter and less frequently in the early spring. The vector ticks of Lyme borreliosis require shade, high humidity in their microhabitat, and a ready access to preferred vertebrate hosts. Lyme borreliosis is the most important vector-borne infectious disease in the United States, accounting for more than 90% of all reported cases in 1991. Lyme borreliosis in humans is a multistage, multisystem, acute and chronic, noncontagious inflammatory condition caused by infection with B. burgdorferi.