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Lyme disease, first detected in Lyme, Connecticut, is a bacterial infection caused by the spirochete Borrelia burgdorferi. It is a zoonotic (animal-to-human) infection within the deer and rat populations and accidentally transmitted to man through the bite of the deer tick, Ixodes dammini. Different arthropod vectors are instrumental in the transmission of other Borrelia infections. The extremely small size (< 2 mm) of deer ticks makes them difficult to detect, compared to the larger wood tick (Dermacentor andersoni) or dog tick (D. variabilis). A large, circular, red petechial rash usually develops at the site of the tick bite. It may grow to over 10 cm in diameter and can become surrounded by an even more extensive halo of less intense rash. The bacteria may cause joint pain (arthlagia) resulting in a form of arthritis.

The primary lesion grows into an enlarged rash as the spirochete proliferates and invades the blood and lymphatic vessels. The lesion usually disappears after several weeks. Some individuals progress into a second stage of infection involving neurologic or cardiac abnormalities. An arthritic response may develop months to years after the initial infection, probably due to an autoimmune response. This is indicative of the third stage of Lyme disease.

Relapsing fever is another disease caused by bacteria of the genus Borrelia. It can be either epidemic (louse-borne) or endemic (tick-borne). Syphilis and leptospirosis are diseases also caused by different spirochetes.