Rop'ngor

Rop'ngor was a Klingon virus which typically afflicts Klingon children at a young age. It attacks the endocrine system, and then enters a dormant state before its host's immune system can produce enough antibodies, and then will re-emerge once white blood cell counts fall again. This behavior earned it the name "cheating disease" in tlhIngan Hol. The virus can also be contracted and carried by Humans, though due to physiological differences, the disease causes only mild symptoms in Humans. 

Lieutenant Worf contracted rop'ngor in 2365, causing him to faint while on bridge duty before being treated. Doctor Katherine Pulaski hid the true nature of his condition from Captain Picard in order to save him from embarrassment. 

During this same time, the rop'ngor virus spread to the inhabitants of Mariposa. The pathogen was discovered by Dr. Elizabeth Vallis, who attempted to genetically engineer it as a way of repairing the long-term damage to the reproductive systems of the Mariposan clone population. Tragically, a mutated strain of rop'ngor escaped in 2376, killing hundreds.