Online multiplayer is also available, and it's fun to some extent. You play as survivors or the infected. If you are a survivor you have to stop any infected players from attacking you and basically see how long you last with your points getting higher. If you are the infected you have to stop the survivors from gaining points, you basically kill them till there is no-one left. After that there is a team swap, so the survivors take a turn at being the infected, therefore the infected are now the survivors and have to try and last longer and get more score... Get it?
So all in all it's a very good game. It's fairly short... but it's one hell of a thrill ride!