Fibrosis in the lung. Light micrograph of lung tissue showing fibrous tissue in pulmonary fibrosis. This is a condition in which the tissue surrounding the alveoli (air sacs) in the lungs, the interstitium, becomes inflamed, leading to the production of scar tissue. This reduces the lungs' capacity for gaseous exchange, leading to shortness of breath and, often, a cough. Pulmonary fibrosis usually occurs following a lung infection, such as pneumonia or tuberculosis. Treatment is aimed at curing the underlying cause. Magnification unknown.