Urticaria, commonly known as hives, is a condition that affects about 20 per cent of the population at some time.
Hives may come in two different forms – the ACUTE, which can last from a few hours to less than six weeks, and the CHRONIC, which lasts for longer than six weeks. The welts may disappear and then reappear after a short time. Scratching these hives will make them worse, so it is important to avoid this natural tendency.
In many cases, no apparent cause is found for hives even after intensive investigation by a specialist may be required.
Allergic reactions to foods, drugs, insect stings, latex, cold air, sunscreens, ultraviolet light and x-ray dyes often cause Acute urticaria.
Viral infections such as the common cold, strep throat, infectious mononucleosis, or hepatitis, could induce this reaction.
In someone who is prone to hives, the triggers will cause the body to release chemical mediators, including histamine, which causes a runny nose and watery eyes. Histamine dilates the blood vessels and allows fluid to leak into the surrounding tissues… the result is itching and swelling. – Allergy Asthma & Immunology Society of Ontario
SOURCE: http://allergyasthma.on.ca/patient/rhinitis-hay-fever/
