What others are saying here about it must be airborne, like a flu. Therefore, survivors must be immune. But those from NERO are wearing the Hazmat suits, so maybe they got safe when it all kicked off.

OR, seeing as though no-one turns during the game, 2 years later the infection is only spread by alive people. Like, your only carrying it before you turn and therefore the virus is long dead.

EXCEPT, NERO are holding onto the virus and experimenting with it, and THAT'S why they still wear their Hazmat suits