Diffuse alveolar damage (DAD) describes a common histologic injury pattern of the lung. The early stages are characterized by epithelial cell necrosis and sloughing, fibrous exsudate, edema, and hyaline membranes made of surfactant and proteins, filling the alveoli. This results in impaired gas exchange. In later stages, type II cells and myofibroblasts proliferate within the interstitium and airspaces. The corresponding clinical entity is acute respiratory distress syndrome (ARDS). DAD may result from pulmonary drug toxicity, occurs in immunosuppressed, severe viral infections, acute interstial pneumonitis, and crack cocaine inhalation. The phenotype is Human Phenotype Ontology term HP_0033006 (Diffuse alveolar damage).