Pulmonary fibrosis. Overview. Pulmonary fibrosis Pulmonary fibrosis scars and thickens the tissue around and between the air sacs (alveoli) in your lungs, as shown on the right. A normal lung with normal alveoli is shown on the left. Pulmonary fibrosis is a lung disease that occurs when lung tissue becomes damaged and scarred. This thickened, stiff tissue makes it more difficult for your lungs to work properly. As pulmonary fibrosis worsens, you become progressively more short of breath. The scarring associated with pulmonary fibrosis can be caused by a multitude of factors.