Hay fever is also known as pollinosis or seasonal allergic rhinitis. For most people, a single allergen sets their symptoms off at about the same time each year. Spring attacks are usually due to tree pollen (not hay), while grass pollens dominate in the summer and weed pollens in the autumn. This can vary slightly by region in Canada. Hay fever is also occasionally cause by airborne fungal spores. Hay fever affects about 20% to 25% of Canadians and the prevalence is increasing.