Definition 
Interstitial pneumonitis is an acute, patchy inflammation of the alveolar walls (interstitium).

Pathology
There is edema and mononuclear infiltration of the interstitium without consolidation. In severe cases, protein rich liquid and hyaline membranes may occupy some alveoli.

Etiology/Pathogenesis
Mycoplasma, variety of viruses, chlamydia.

Pathophysiology
Severe upper respiratory tract infection or "chest cold" along with fever, headache, myalgia.