To earn this achievement you must play an online Pursuit Knockout race with a person who already has this achievement. It doesn't matter if it's a ranked or player match or if you win or lose.
Keep in mind, the viral achievements are a bit buggy and may not always unlock after just one race. Sometimes you'll need to race against an infected player more than once for it to pop.
Believe it or not, there are still people playing this game despite it's age. However, it may be difficult finding someone who has the viral achievements. If you're having trouble, I can help you get this achievement. Just shoot me a message on Xbox LIVE.