@comedyxtragedy it depends.
in games like left4dead and resident evil, the zombies aren't actually dead people. they just have limited brain activity and pretty much still alive, they still can see, hear, or smell and know you're not one of them and they don't question it.
in games like last of us, the body is pretty much dead and the infecting disease actively search for uninfected host. just like that ant-fungus thing, except the target is not high-ground but another human.
in fantasy fiction, whatever the coolest reason is taken because rule of cool.