Erythema multiforme (EM) is a serious inflammatory skin eruption (rash) caused by allergic reactions to many different things, including prescription and over-the-counter drugs. It can affect people of all ages and is often more severe in children and young adults. Erythema multiforme often starts as a red rash on the palms, soles, and back of the hands. It can spread to the trunk, face, and mouth in severe cases. Some people only have erythema multiforme in the mouth.

A number of published studies have shown that the proportion of diagnosed Lyme disease patients who present the characteristic rash erythema migrans (EM) is within the range 85 to 95%. The journal Clinical Infectious Diseases (Krause 2002) examines patients suspected of early Lyme disease, babesiosis, and/or anaplasmosis. Among confirmed Lyme disease patients who are not coinfected with other tick-borne diseases, 90% exhibit an EM.