^^^ Hm, but these gangsi don't die if decapitated, only if their heads are crushed. Unless they cut their head off faster than the disease can get to it, I suppose?
Plus, if someone died to a Gangsi bite, normally they would probably still be under assault from a gangsi, unless they're some kinda master that killed all nearby gangsi and retreated after getting a small injury or something.
And even if they're all martial artists, it's not as if all of them would decisively decapitate themselves as soon as they're infected.