It is confirmed that Modern Warfare has lobbies that are hacked. It basically has two modes which is Slow Motion and Super Speed.
For now, it is only confirmed to be on the Xbox 360 only. PC and PS3 are still unconfirmed for the time being.
You can get infected if you join a hacked lobby and when you leave you will be carrying the infection. The only way for you to infect others is to host a lobby.
For a more thorough explanation, head on over to MP1st.