a pulmonary disorder that results from the development of hypersensitivity to inhaled dust from moldy hay or other fodder. In the acute form, symptoms include a sudden onset of breathlessness, fever, a rapid heartbeat, cough (especially in the morning), copious production of phlegm, and a general sense of feeling ill. Attacks may last a few days to several weeks. In its chronic form, farmer's lung may persist for years and lead to respiratory failure from chronic bronchitis or pulmonary fibrosis. Avoidance of offending dusts is the most effective treatment.

Farmer's lung is the prototype of a number of diseases that are categorized as hypersensitivity pneumonitis; these include pigeon breeder's lung (also called bird fancier's, or bird breeder's, lung), mushroom worker's lung, cheesewasher's lung, and coffee worker's lung.