It's a reaction to hay which is a type of grass. It's what you get when you cut the grass and don't react well. Hay fever is a low-grade fever. Your body mounts a little weak defense like a viral defense, but instead of getting a high fever, you get a low-grade fever. There are two types of named allergy fevers. One is hay fever. It typically occurs in the fall in hay season. The other is cedar fever in the south, particularly Texas. Cedar trees go berserk in January and February. People get a temperature and we call that cedar fever. When someone mows the grass around you, it stirs up a bunch of pollen and mold out of the yard. That's why you get hay fever or an allergic reaction to newly mowed grass. Some kids are so allergic that if they roll on the ground, they break out in a rash.
