Acute Interstitial Pneumonia (AIP) Overview: Acute interstitial pneumonitis (AIP), also known as Hamman-Rich syndrome is an uncommon type of idiopathic interstitial pneumonia for which researchers have not yet found a cause (idiopathic). Pneumonia results when tissues of the lungs become inflamed in reaction to infection. The inflammation can also extend into the bronchioles, the small airways that branch off into the lungs. Prolonged inflammation causes fibrosis. Over time, scar tissue causes the bronchi and the walls of the bronchi widen, or are destroyed, resulting in the lungs shrinking. The condition usually progresses to respiratory failure and medical treatment is immediately required.