Zombies are not dead people with out brains, zombies have brains, its what keeps them alive, does tha know nothing about zombies?
Zombies are dead people that are still walking around with the basic instinct which is feed, the power for zombies is that some kind of reaction, viral or other, created an electrical charge which re-awakened the brain and re-animate the body, the only way to kill a zombie is kill the brain.
But we are on KF here and we are talking about specimens, its a story that confused me too, if they're experiments, why are there thousands upon thousands of them, surely from all then killing everyone has done over the space of 20+ maps would have wiped them out, and as for patty, well you can blow him up, burn him, chop him and yet he still comes back stronger than ever every game, KF has a slanted story which has some very dodgy edges.
BUT! They are known as Specimens, although I would call a clot a zombie, but I wouldn't call a crawler one.