Infected, also known as Sickness as an April Fool's day joke, is a custom game mode introduced in Call of Duty: Modern Warfare 3, which started as a private match game mode before being released to the online community public playlist on February 5, 2012 for PlayStation 3 and Xbox 360, and on May 9, 2012 for PC.
Defender is the closest thing Call of Duty: Modern Warfare 2 has to a traditional Survival mode, and is basically a combination between that style of gameplay and the Sniper Fi mission from the original MW2's Spec Ops.