Fibrosis of the lung. Light micrograph of a section through a lung showing fibrous (scar) tissue (black). The fibrous tissue is a result of inflammation of the tissue surrounding the alveoli (air sacs). The fibrous tissue reduces the lung's capacity for gaseous exchange, leading to shortness of breath. The condition may be caused by an infection, inhalation of irritants, or certain medications. Treatment is aimed at the underlying cause.