No "Infection" is on Halo 3 and its were one person is a zombie and has an energy sword or what ever desired weapon you make the zombie have and make them fast slow cant jump high can jump high.
And when a zombie kills you, you become the zombie or as i said you can change settings so you can make it you join them so yer like a last man standing game just gets harder and harder.
And it is not on Halo 2...Thats a myth well from what i know.
Uh infection is when you (I think) you go against wave after wave of Flood infection an combat form until everyone is dead.
Wma ahahahahahahahahahah