pneumonia (n.) Look up pneumonia at Dictionary.com
c.1600, from Modern Latin, from Greek pneumonia "inflammation of the lungs," from pneumon "lung," altered (perhaps by influence of pnein "to breathe") from pleumon "lung," literally "floater," probably cognate with Latin pulmo (see pulmonary), from PIE *pleu- "to flow, to swim" (see pluvial). Alteration in Greek perhaps by influence of pnein "to breathe."