Easiest way to get this is with a friend if possible. Start a Private Online Freeroam session, invite a friend who is prepared to join you, navigate to one of the playground game icons (such as the 4 at the air field). Then start a game of infected, and whichever of you starts out infected hit the other (or alternately, just play out the clock).
Of course this can also be done in a public online session (again, online freeroam is best to avoid having to do a whole set of playground games). Also the other playground games (King and Flag Capture) will work, but "Infected" is just quicker, completing in a few seconds under best circumstances.