Hypersensitivity pneumonitis: a type of inflammation of the lung that develops from breathing in a foreign substance. Molds (fungi) or dust are the most common substances that cause this reaction. Symptoms vary according to severity and can include shortness of breath, coughing, and fever, and chills. The symptoms may develop within 4-6 hours after inhaling the offending substance, or, the symptoms may be chronic in cases in which there is continuous exposure to the agent.
The most common type of hypersensitivity pneumonitis is Bird fancier's lung, caused by repeated exposure to proteins in the droppings or feathers of birds. Farmer's lung is another type of hypersensitivity pneumonitis that is caused by exposure to dust from moldy hay, straw, and grain. The condition can also develop as a result of chemical exposures or exposure to organisms that contaminate heating or air conditioning systems.