Urticaria or hives as it is commonly called, is an itchy rash consisting of localised swellings of the skin that usually last for a few hours before fading away. Urticaria results from changes in the small blood vessels of the skin. Such changes are brought about by the release of substances in the body, the commonest of which is histamine.
Acute urticaria, which can last from a few hours to as long as a week is usually caused by drugs, specific foods or a viral infection. Sometimes, no cause can be detected. Urticaria occurring almost daily for more than six weeks is called chronic urticarial. It can be spontaneous (no inciting factors) or inducible (triggered/aggravated by physical factors such as heat, pressure, cold, vibration, sweat and sunlight etc).