guys in the tanks probably picked up wounded comrades who later bit them, Avatar was only meant to be watched on IMAX 3D, any other way it is incredibly clichéd boring tripe.
Speaking of Zombies, I had a dream the other night that I was one! Very strange, I was in a doorway because I knew that I had lived there before and hid behind a door to bite an old friend or family member ( didn't know who exactly but hey, I was a Zombie it was understandable) I bit them on the head, tasted like chicken. mmmmmmm brains