As of posting, it is only possible to get this achievement in rumble pit. Keep playing until infection gametype appears on a mythic map. If you start as a human, to get this achievement you need to be infected so run towards zombies and do not kill them and they will infect you. Once a zombie, you have to infect two other humans. In infection humans have limited radar so staying out of line of sight in essential and once close enough move in for the kill quickly before they have time to react to the red dot on their radar.
I would also advise going for the zombie repeller achievement in the same game.
This achievement will be easier to earn in the living dead double experience playlists.
This achievement can only be unlocked in rumble pit if all the players have the mythic pack, so i would advise playing with friends who also own the mythic map pack.