Lyme disease is caused by the spirochete Borrelia burgdorferi. After B. burgdorferi is introduced into the skin, it spreads locally. The local spread leads to erythema migrans (EM), a rash that is found in approximately two-thirds of patients. Keep in mind that a rash may not be present in up to 20% of cases and even if present may be mild, nonspecific, and easily missed by the patient, family, caregivers, and/or the initial treating physicians.