Wherein Klingons are attacked by a powerful, biological weapon...
You know, it wouldn't surprise me in the least if the Tribbles were actually an engineered biological weapon. They're almost literally virii on a macro-scale. It just so happened that the instruction manual and warning labels were lost, and Whatsisface (Mudd?) thought he could make a profit selling them as pets. So, Neglectful Precursors and Forgotten Superweapon?