They appear, sometimes in clusters, on the face, trunk of the body, and, less often, on the scalp, hands, or feet.
Hives can change shape, fade, then rapidly reappear.
A single hive lasts less than 24 hours. After an attack, though, new ones may crop up for up to 6 weeks. According to estimates, nearly 20 percent of Americans will get hives at some time in their lives. Sometimes it is not known what causes hives.