There are various cheats that will "infect" your system for a time as long as you don't go back to the dashboard or shut off the system. They can do things like aim-bots or super speed running. These are activated when you are killed by someone else who is already infected. Some can be fun, but like all cheats, some make the game unbearable and almost impossible to play like all shots are head-shots or Invincibility. These "infections" are most prominent in all the CoD's but have become less popular due to Activision actually doing something about it.
Gamer No. 1:(Playing CoDMW2) Dude I just got infected, now I got SuperSpeed!
Gamer No. 2: Man FUCK Infections. I keep getting SpawnTrapped GodDamnit!!1