Pityriasis (pit-ih-RIE-uh-sis) rosea is a skin rash that usually begins as one large circular or oval spot on your chest, abdomen or back. Called a herald patch, this initial spot can be up to 4 inches (10 centimeters) across.
The herald patch is typically followed by a distinctive pattern of similar but smaller lesions that sweep out from the middle of your body in a shape that resembles drooping pine-tree branches.
Pityriasis rosea can affect any age group, but it most commonly occurs between the ages of 10 and 35. It usually goes away on its own within six weeks. Pityriasis rosea can cause itching, and treatment usually focuses on relieving symptoms.