Rop'ngor was a Klingon childhood disease that was comparable to Human measles. Fainting could be a symptom of this illness.
Despite not being a child at the time, Worf was afflicted with this illness in 2365. Doctor Katherine Pulaski hid the true nature of his condition from Captain Picard in order to protect Worf's reputation. (TNG: "Up The Long Ladder")
