Hives also known as utricaria are common and affect more than 40 million people in the U.S. They present themselves as welts on the skin, occurring individually or clustering together to form one large welt. Hives are most commonly caused by allergic reactions, although they may also be evidence of an infection or underlying illness. Some people even develop hives as a result of stress or exposure to the sun. Hives are generally not dangerous. However, persistent, unexplained hives require further examination, laboratory work evaluation and allergy testing.