Allergies occur when the body's immune system reacts to a normally harmless substance. They are extremely common, affecting around 50 million people in the United States.
Many substances that enter the body or touch the skin can cause an allergic reaction. It is estimated that 35 million people are allergic to substances found in the air. About 6 percent of children under the age of 6 and 2 percent of adults are allergic to certain foods; and 3 percent of hospitalized patients are allergic to certain medicines.
Allergies can strike anyone, regardless of age, gender, ethnic background, or social or economic status. They are the sixth leading cause of chronic disease in the United States, costing the healthcare system 18 billion dollars each year.