Arvid Afzelius first described and named erythema migrans (EM), a clinical entity that he assumed to be caused by an agent transmitted by the bite of a tick (Ixodes reduvius). Certain neurologic, cutaneous, and other syndromes observed in Europe were recognized in the 1920s and 1930s to be disabling sequelae of EM. In the 1940s and 1950s the effectiveness of penicillin as therapy for EM was demonstrated. In 1968 the first patient with EM and neurologic sequelae in North America benefited from treatment with penicillin. In 1975, an epidemic arthropathy appeared in the area of Lyme, Connecticut. Despite resemblance to EM (the initial appearance of cutaneous lesions), the complex was called Lyme disease because of the occurrence of cardiac, neurologic, and arthritic sequelae. The vector of Lyme disease, Ixodes dammini--a tick that harbors agents that cause Lyme disease and babesiosis--was identified and characterized in 1979. The spirochete that causes Lyme disease was designated Borrelia burgdorferi. The North American and European species of spirochete and the clinical syndromes to which they are related are described.