What are some reasons a that a person breaks out in hives?

Food allergies,other: Food allergy, drug allergy are common causes of hives. Patients who are very allergic to grass pollens or animals can sometimes develop hives when touching them. Rarely, people with internal diseases, such as viral hepatitis, autoimmune diseases, HIV can have hives. There are also people who have hives for no reasons. Their hives can be triggered by heat, pressure, menstrual periods and stress.