Allergies affect more than 50 million Americans, or one in every five adults and children, and are as common in men as women. Thirty-five million people suffer from upper respiratory symptoms that are allergic reactions to airborne pollen; up to 10 million Americans are allergic to cats; and two million develop severe allergic reactions to insect stings.

Food allergies are less common, affecting about 12 million Americans. While up to one out of three people say they have a food allergy, only about one to two percent of adults and six percent of children have true allergic reactions to foods. Unfortunately, about 150 deaths per year are attributed to food allergy.

Allergies have a genetic component. If one parent has allergies, each child has a 30 to 40 percent chance of having an allergy. If both parents have allergies, each child has a much higher chance—75 percent—of having allergies.

Adults usually do not lose their allergies, but children can sometimes outgrow them.

An allergy is your body's response to what it perceives to be a threat. Your body attacks the allergen in a way similar to how it would attack an invading virus or bacterial infection.

The pollens of some plants, grasses and trees are so small and light they can travel through the air for miles. Samples of ragweed pollen have been collected 400 miles out at sea and two miles up into the atmosphere.

A pollen count represents the concentration of all the pollen or of one particular type, like ragweed, in a certain area over a certain period of time; it is usually expressed in grains of pollen per cubic meter of air collected over 24 hours.

Molds, too, are light and can be easily carried by air currents. The mold season peaks in July in warmer states and October in colder ones, but mold can thrive year-round indoors.

It's the waste product of dust mites, not the mites themselves, that cause the allergy. Similarly, the waste products of cockroaches are an important cause of allergy symptoms, particularly in some urban areas of the United States. Because dust mites can't be drowned, the only way to get rid of them in washable items like rugs and curtains is to use water that is hotter than 130 degrees Fahrenheit and a hot dryer setting.

The major allergen from animals is not their fur, but proteins secreted by skin glands that are found in dander; proteins in the saliva that stick to fur when the animal licks itself; and proteins found in the animal's urine.