When hordes of Infected appear, find a room with a door and no windows. Go inside and close the door. The Infected might break the door, but when you see a hole in it, shoot the Infected breaking it down. Your teammates can handle most of the Infected.
When low on health and alone, allow the CPU controlled players to attack Witches and Tanks. This will keep you from dying and also weaken enemies. The CPU characters will later appear behind a closed door with 50 health.