Lyme disease is an infection caused by a kind of bacteria (germ) called a spirochete. This bacterium, Borrelia burgdorferi, is transmitted by the bite of an infected deer tick. It is the most common tick-borne infection in both North America and Europe. Diagnosis is based on a variety of symptoms, physical findings, blood tests, and a history of exposure to infected ticks. The June 20, 2007, issue of JAMA includes an article that discusses a rash called erythema migrans (see below) as a clinical sign that is consistent with a diagnosis of early Lyme disease and other related tick-borne diseases.