Erythema migrans is the initial lesion of Lyme disease, and often appears at the site of the infecting tick bite. It is a red, enlarging rash, flat or slightly raised, and may reach from 4 to 20 inches across (the average rash is 6 inches). (Image courtesy of the Centers for Disease Control and Prevention.)
Update Date 2/3/2014
Updated by: Jatin M. Vyas, MD, PhD, Assistant Professor in Medicine, Harvard Medical School; Assistant in Medicine, Division of Infectious Disease, Department of Medicine, Massachusetts General Hospital, Boston, MA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.