An allergy is an adverse reaction of the body's immune system to a substance that is harmless to most people. For some, having an allergy is not a "real" disease. However, more and more people, adults and children, live with some type of allergy. The World Health Organization classifies allergy as the 4th disease in the world. Food allergies have doubled in the past decade and asthma, rhinitis and eczema have tripled in the past 20 years. An allergic reaction can lead to a variety of symptoms, such as sneezing, wheezing, nausea, itching, red eyes, most of which are mild and not life-threatening. However, a small number of people can develop severe allergic reactions (anaphylaxis) that can be life-threatening. Allergies are not always easy to diagnose, especially in children. Diagnosing and controlling allergies helps restore quality of life and is done by allergists.