Hives, also known as urticaria (urtica = nettle) is a rash, often fiercely itchy, that develops in a very short period of time and can cover the entire body. The existing blisters disappear within 24 hours, but are substituted by new patches. The skin abnormalities can merge into larger patches, also known as urtica.
Urticaria is a reaction of the immune system to an unknown stimulus. Such a stimulus can be an infection, medication or a reaction to food. The underlying cause is usually not determinable. Urticaria is, however, well treatable, calming the skin.