A multisystem disorder caused by the tick-transmitted spirochete Borrelia Burgdorferi.

After an incubation period of three to 32 days, usually in the summer when ticks are most active, Stage 1 begins with a characteristic expanding skin lesion, called erythema migrans, at the site of the tick bite.

Stage 2 begins within the next few days to weeks. The spirochete may spread to other skin sites and to the heart, joints, and nervous system.

Stage 3 begins months after the onset of the infection.

In patients who have not received an antibiotic, 60 percent will develop arthritis.