An allergy is an overreaction by your body's immune system to a normally harmless substance, or allergen, that does not bother most people. According to the National Institute of Allergy and Infectious Diseases, allergies are among the major causes of illness and disability in the United States, affecting as many as 50 million Americans. From 10% to 30% of adults and up to 40% of children have allergies. Allergy symptoms may significantly affect your quality of life by causing fatigue, headache, and difficulty in daily functioning.