Every year, about 20% of Americans will get hives—those itchy, red bumps or welts that can appear after a day in the garden, taking medication, being bitten by a bug or for no apparent reason at all. Patricia A. MacCulloch is a nurse practitioner and professor of nursing who teaches about hives, among many other things. She offers some insight into this annoying condition that can sometimes be a sign of a life-threatening emergency.