Hypersensitivity pneumonitis is usually an occupational disease in which exposure to organic dusts, fungus, or molds leads to acute and then, over time, chronic lung disease. Exposure may also occur in the home, from fungus present in humidifiers, heating systems, and air conditioners. Some people may have hobbies that can lead to exposure, such as owning birds.

Acute illness may occur 4 to 6 hours after the exposure, once the person has left the area where the allergen is present. Chronic illness with changes seen on chest X-ray may develop with continued exposures. The chronic form of this disease may lead to pulmonary fibrosis (a scarring of the lung tissue that is often not reversible).