Disorder in which fibrous connective tissue spreads on or replaces normal smooth muscle or other normal body tissues. Fibrosis is more common in the heart, lung, peritoneum and kidney. PULMONARY FIBROSIS: Scar tissue formation in connective tissue of the lungs as a sequela of any inflammation or irritation caused by an infectious or carcinogenic process. Localized fibrosis may give rise to different complications such as pulmonary infarction, abscess, or bronchiectasis.