chronic progressive form of pneumonia with wheezing, dyspnea, and diffuse infiltrates seen on radiographs; occurs following exposure to any of a variety of antigens, sometimes occupational, and many names are given to cases with known types of exposure (such as farmer's lung, maple bark stripper's lung, chicken plucker's lung, bagassosis, byssinosis, and humidifier lung); biopsy findings usually show patchy infiltration of alveolar walls with lymphocytes, plasma cells, and other inflammatory cells; can progress to irreversible interstitial fibrotic disease with restrictive pattern on pulmonary function, but in early disease most manifestations are reversible if offending antigen is identified and removed from environment.