Hay fever is an allergy that occurs most frequently during the spring, summer, and fall. Grasses, trees, and weeds produce pollen during those seasons, and hay fever sufferers are allergic to pollen. About 6 per cent of the people in the United States have hay fever. Like most allergies, hay fever runs in families. A person can develop the allergy at any age. The medical name for hay fever is pollinosis. See Allergy.