343 revealed that Infection mode will be returning to Halo when it makes its return in Halo 5 Guardians later this year.
In a livestream detailing the final details of the soon to be released Ghosts of the Meridian update last night, the developer revealed that Infection mode will be coming back, but did not provide any specific time frame. Infection is more a fun rather than serious version of multiplayer, with one player acting as a zombie. Your goal as the zombie is to infect as many other players as possible and was last seen in Halo 3 and Reach. It most likely will debut in the update after Ghosts of the Meridian as that is how 343 has been releasing their updates so far.