The Infection game modes requires a minimum of 4 players to start and play a match. Most people that are playing this under-rated game are usually boosting, using the method described @ x360a.org:
"The best way to boost this is have the infected player grab a shotgun and stand at the enemy spawn on Court. They only spawn to the left or right of this section so you can kill them almost instantly when they spawn. Taking turns with four players you will net about 100 points every 45 minutes. Setup a 60-min match and have one person get 95-98, then the next, then the next and the last person can finish to 100. Rotate who gets 100 to be fair."
Once 500 points has accumulated, the cheevo will pop at the end of the match.