The term "bronchiolitis" generally refers to inflammation of the bronchioles.[1] DPB is classified as a form of "primary bronchiolitis", which means that the underlying cause of bronchiolitis is originating from or is confined to the bronchioles.[2][3] Along with DPB, additional forms of primary bronchiolitis include bronchiolitis obliterans, follicular bronchiolitis, respiratory bronchiolitis, mineral dust airway disease, and a number of others.[2] Unlike DPB, bronchiolitis that is not considered "primary" would be associated with diseases of the larger airways, such as chronic bronchitis.[2][3]