catarrh (n.)

word-forming element meaning "down, downward," but also "through, on, against, concerning," etc., from Latinized form of Greek kata-, before vowels kat-, from kata "down, downward, down from, down to," from PIE *kmt- "down, with, along" (source also of Hittite kattan (adv.) "below, underneath," katta "along with"). Occasionally in Greek it had senses of "against" (catapult) or "wrongly" (catachresis), also "along, through, over, across, concerning." Also sometimes used as an intensive or with a sense of completion of action (catalogue). Very active in ancient Greek, this prefix is found in English mostly in words borrowed through Latin after c. 1500.

Proto-Indo-European root meaning "to flow."

It forms all or part of: amenorrhea; catarrh; diarrhea; gonorrhea; hemorrhoids; maelstrom; rheo-; rheology; rheostat; rheum; rheumatic; rheumatism; rheumatoid; rhinorrhea; rhythm; seborrhea; stream.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit sravati "flows," srotah "stream;" Avestan thraotah- "stream, river," Old Persian rauta "river;" Greek rhein "to flow," rheos "a flowing, stream," rhythmos "rhythm," rhytos "fluid, liquid;" Old Irish sruaim, Irish sruth "stream, river;" Welsh ffrwd "stream;" Old Norse straumr, Old English stream; Lettish strauma "stream, river;" Lithuanian sravėti "to trickle, ooze;" Old Church Slavonic struja "river," o-strovu "island," literally "that which is surrounded by a river;" Polish strumień "brook."