The medical term for hay fever is allergic rhinitis. While hay fever symptoms are similar to cold symptoms, a cold is caused by a virus while hay fever is caused by an allergic response of your immune system. Some people suffer from hay fever all year round, and some suffer only during certain seasons. Medically, these conditions are known as seasonal rhinitis and perennial rhinitis. Around one in five people suffers from at least one kind of hay fever.
Seasonal rhinitis is what most people call hay fever; it's an allergy that's related to the reproduction of plants and fungi. As trees, grass and weeds release their pollen to fertilize each other, you inhale the airborne particles and your immune system overreacts. Instead of properly registering the pollen, your body thinks that it's in danger and it sends out an antibody to fight off the invaders. The antibody then triggers other chemicals in your body, including histamine, and you suffer the allergic consequences. Pollen allergy-related hay fever is typically a problem anytime from spring to fall, depending on what you're specifically allergic to. Meanwhile, a fungus-related form of hay fever can last a lot longer, since fungi release their spores from spring to late fall.
In contrast to seasonal rhinitis, perennial rhinitis can last year round and is the result of an allergy to other airborne allergens like dust mites, pet dander and mold. These allergens can be found all over the house, and without vigilant cleaning, they can build up in your bed, on your curtains, on your couch cushions and in the fibers of your carpet. Some people suffer from both perennial and seasonal hay fever, but often the perennial hay fever overpowers the seasonal one and you don't even realize you have it.