Ok i found a bug thanks to a friend on our server. Well you ever remember those Zombie Nades? Well this bug is reverse. If your a human and throw a nade at someone right after you turn and it lands by a human they die instead of zombified. we TEsted this and it happens everytime if the person is low enough health.
I dont think this can be fixed. Before the first zombie appear, no one can hurt anyone. After the first zombie appear, you can damage the other team. How can humans damage zombies if zombies cannot damage humans? This is not a bug. Its how css works.