It is a super-sensitivity to a substance that is not harmful to most other people. These substances are called allergy triggers or allergens.
Some people also have food allergies.
As many as 50 million Americans suffer from allergic rhinitis, the most common allergy illness. Almost half of allergy sufferers, about 25 million people, have symptoms at least 4 months of the year. Of these, about 10 million have symptoms 9 months of the year. More children than ever before have allergies. In fact, children miss 2 million school days a year because of allergies and almost one of every three of their healthcare office visits are for treatment of allergy symptoms.