Systemic and pulmonary reactions resulting from inhalation of dust from moldy hay, threshing dust, or moldy straw, by persons who have become hypersensitive to antigens in the dust; most often associated with inhalation of spores of micromonospora faeni or thermoactinomyces vulgaris.

Farmer's lung (not to be confused with silo-filler's disease) is a hypersensitivity pneumonitis induced by the inhalation of biologic dusts coming from hay dust or mold spores or any other agricultural products. It results in a type III hypersensitivity inflammatory response and can progress to become a chronic condition which is considered potentially dangerous..