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Lyme disease, a potentially debilitating illness in humans if untreated, is caused by the spirochete Bonrelia burgdorferi. First recognized in the mid-1970s in Lyme, Connecticut, the disease has been reported in at least 46 of the United States and in Canada, Australia, Asia, and Europe. This disease is primarily transmitted to humans by tick bite. In California, the tick believed primarily responsible is the western black-legged tick, Nodes pacificus In the upper midwestern and northeastern states, the usual vector is Nodes scapularis (formerly I_. ammini in part).