Creature
The zombies in The Crazies are interesting. They “turn” while they’re still alive. They behave the same as an undead zombie as they did when they were a living zombie. While they’re not sane, they retain some of their human qualities (talking, movement, intention, ability to wield a weapon). However, the infected become mass murderers.
They are not in it for the human flesh. They’re in it for the numbers. Hence, the movie title.
They’re undead but don’t seem to have any of the zombie hallmarks of being hard to k*ll.