Rop'ngor is a Klingon illness that is comparable to Human measles. It is typically experienced at a young age by Klingon children. Fainting can be a symptom of this illness.

Despite that, 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")