The difference between the two is that pulmonary fibrosis is idiopathic unexplained diffuse interstitial lung fibrosis, a serious lung disease, whereas pulmonary fibrosis foci are caused by an inflammatory response in the lungs, such as due to infection with viruses, bacteria, fungi, Mycobacterium tuberculosis, etc., and are scarring left behind after an inflammatory response in the lung mucosa, manifesting as fibrotic lesions in the lungs of varying degrees. In contrast, pulmonary fibrosis is more pronounced, more widespread and generally diffuse in distribution, while pulmonary fibrosis foci are relatively small in extent and confined to the site of the inflammatory response.