Hives are usually the result of an allergic reaction to food, medications, infections, or other common allergens, but can also be caused by a chemical irritant, sun overexposure, stress, excessive sweating, or other diseases such as lupus.
The lesions, called wheals, are characterized as red, itchy, raised welts. Hives tend to appear quickly and resolve quickly. They will rarely require medical attention as they usually resolve without treatment.
Hives lasting more than six weeks are known as chronic urticaria or, if there is swelling below the surface of the skin, angioedema. There are no known causes of angioedema, but it can affect internal organs and therefore does require correct diagnosis and treatment, and follow up with a primary care physician.