https://www.gamespot.com/articles/call-of-duty-black-ops-3-update-may-hint-at-black-/1100-6457199/

Call of Duty: Black Ops 3 is well over two years old now, but Treyarch has just issued a patch on PC, PlayStation 4, and Xbox One, adding a free new map and game mode to the 2015 release. The extras are returns of some fan favorites, but the conspicuous timing seems likely to signal clues for the next Call of Duty installment.

The mode is Infected, which hails from Modern Warfare 3 and has appeared in several Call of Duty games since. One player is selected as the titular infected, with the goal of infecting all the other players. If a player is downed, they respawn as infected, making the horde grow larger. The infected team wins if they successfully convert all the players. If one or more players manage to survive until time runs out, they win instead.