A port of an old gamemode for Timesplitters: Future Perfect called Virus. The gameplay's not quite the same, having been updated for the latest unreal engine, but the spirit's still fully alive. If you are unfamiliar with the gamemode, it starts with a number of human players and (currently, although this might be changed for balance) one infected palyer. If the infected player touches a human, the human also becomes infected. The game is over either when all the human players are infected or when time runs out (although from personal experience it's very hard to survive even to UDK/UT3's minimum time limit of 5 minutes). I originally made it for PS3 mainly, and I make sure to keep a PS3 build of it live as much as possible. Playing with the maximum amount of players is highly recommended.
The UT3 build of Outbreak. This is recommended if you have both UDK and UT3, because it allows access to more content.