First you need to somehow join a modded lobby like this:
Then you have it, and you can keep it if you are the host of a game. You lose it if you quit Modern Warfare 3, or if you join a non-modded game.
HOW TO GET IT BACK:
If you lose it, all you have to do is find the game in Theater Mode that was modded.
Then after you play it in Theater Mode, you have the infection again!
You may want to place the game in your vault, and name it according to which hack it is.
EDIT: This is not fake. Listen to the people talking.
EDIT #2: Gotta love the fanboys in this thread