Assassin's creed Brotherhood: Infected is a DLC or a standalone game that has a different campaign and weapons to Brotherhood. The story is that whilst fighting Rodrigo with the apple of Eden his piece of Eden and yours combine and accidently shows you a paralel universe where a deadly plague grips Rome turning many people into Zombies.
Camapaign[]
After being transported Ezio walks into Rome and sees it in a terrible state when suddenly three Zombies attack you. After dispatching them you must reach the Tiber island hideout