The Halo Waypoint site has all the details on Memories of Reach, which includes Halo 5's version of the fan favorite game mode Infection:
Surviving just got a whole lot harder. If you die by an Infected hand, you join their ranks. One of Halo's most beloved and frantic modes is back in a whole new way. Band together with your fellow survivors or hunt your prey with cunning and skill; the highly anticipated Infection game mode is making its way to Halo 5! In Infection, 12 players face off in a round-based game mode, where the initial Infected player aims to convert the survivors into Infected via elimination.
In addition, Memories of Reach adds the new Arena map Stasis, along with a lot of new REQ packs for armor, weapons, attachments and more. Microsoft plans to release at least one more free DLC pack for Halo 5 in June.