To get this achievement, you must first get an Infected bonus scene. You get this in one of the 12 hunt scenes, usually around the middle of the game, and only occurs once a game, so if you don’t get it you will have to set up a new game.
Set up an arcade hunt game with the following settings:
Film Set: Which ever map you’re most familiar with.
Game Type: Hunt
Game Size: Any, doesn’t matter.
Round Length: 2 Minutes
AI Hunters: 1
AI Difficulty: Easy
Once you get to the Infected bonus scene you must become infected, then in the last 5 seconds of the round, walk into them to pass on the infection. If you’re not, then wait for the AI to come and infect you. If he does follow them around, infect them in the last 5 seconds. If you start infected or the AI kills you, you can find them by using the big arrow the points to the closest uninfected person to you. Since there’s only one, they may be far away from you. Because you will need to catch up to them in the last 5 seconds of the round make use that you restore all of your speed (sleep) either before the round or at the beginning of the round.