The super-popular Infected playlist has been (or will shortly be) added to the public playlists on Call of Duty: Modern Warfare 3, meaning that everybody can now play the game without playing in a private room – excellent news for those of us who don't usually have friends playing Modern Warfare 3.
Infected splits the group into two teams, infected and survivors. The infected team starts off with only a few people and must kill the enemy team in order to "infect" them. As the infected team gets bigger, the survivor's struggle increases and it gets harder and harder to be on the winning team.
No word yet on if other game modes will be being upgraded to the public playlists, I'm sure there's good reason for each of them to be available and we'll hear more in the not too distant future. And don't forget, new maps are now available via Call of Duty Elite.