This "Infection mode" will cause your character in Retribution, former British Marine James Grayson, to become infected with an alternate mutated form of the Chimera virus Nathan Hale is infected with. You gain a new uniform, R2 weaponry, additional abilities, access to new Chimera areas, and more. Even the story will be slightly altered -- characters in the story will be aware of your Chimerian infection which will alter their behavior accordingly. For more details, Andrew has written a hands-on over at Joystiq that you can access right here.
Source -- Joystiq
Source -- PS.Blog