The worst thing most people suffer from eating is a wicked case of indigestion. But for some, eating the wrong thing can cause hives, swelling, vomiting and even death. As many as 12 million Americans have a food allergy, including 1 in every 13 children under the age of 18, according to the Food Allergy Initiative. Food allergies occur when the immune system mistakes a food protein as a threat and attempts to protect the body by releasing histamines and other chemicals into the blood. A food allergy is different from food sensitivity or intolerance, which does not involve an immune system reaction. You can be born with a food allergy, or it can develop over time. Eight foods are the root of 90 percent of all food reactions, according to the Mayo Clinic. Want to know what they are? Read on. (Text: Katherine Butler)