Pityriasis rosea is a harmless skin disease that causes scaly patches that sometimes itch over the torso, neck, arms and legs. Anyone can get it, but it is most common in people ages 10 to 35.
Typically, the disease begins with the appearance of a single large, pink, scaly, oval patch, called the "herald patch." This patch is approximately 1 inch to 2 inches long. It tends to have a salmon-colored center surrounded by a darker pink ring, which some people mistake as a sign of ringworm.
Within about two weeks, many smaller patches appear, sometimes hundreds of them. They appear mainly on the abdomen and back. On the back, the patches tend to line up along the ribs, giving the rash a distinct "Christmas tree" pattern. The rash goes away on its own, usually within two months. Any skin discoloration from the rash will fade after another few months.
No one knows what causes pityriasis rosea. It seems to occur most often in spring and autumn. It is not thought to be contagious, and it does not lead to any other disorder.