Morbilliform rash is the most common cutaneous drug eruption, and is characterized by a measles-like viral exanthem. Morbilliform reactions usually start in dependent areas and then become generalized. They usually begin 2 to 10 days after initiation of the drug, but can occur up to 14 days after a drug is stopped. The immunologic mechanism involved in this type of rash is not known. Morbilliform rash is frequently related to antimicrobial therapy. For example, morbilliform rash is seen with high frequency in patients with Epstein-Barr virus or cytomegalovirus mononucleosis treated with ampicillin and in patients with human immunodeficiency virus (HIV) infection who are treated with sulfa drugs.