How Modern Warfare 2 Infection Lobbies Work
When the original Playstation 3 and Xbox 360 consoles were sold, some owners decided to modify them by swapping out hardware or running certain types of firmware to modify their system in certain ways. Some attached external storage devices to increase the amount of information their consoles could contain, while others found ways to install all the information off a game disk to play the game without using the disk at all. (The larger data storage only increasing the amount of games they were able to save.) Some others figured out that by messing with the right bits of information they could modify their own games, and this is basically the process of how Modern Warfare 2 Infections do their thing.
How Infections Are Spread
Someone with a modified system and program or code creates a private game and invites people to play. Not only does the person who modified the game originally benefit from the glitches, but as well as everyone else who plays within that private game. From this point everyone who has played in that private match can make additional private games and continue spreading the 'infection' along, assuming they are the host of the match. These modified effects last for as long as you play the game in private matches. The moment you go to a rated or public match these infections are lost, only remaining as long as you keep your system turned on.
A Word of Caution
All modifications to any console or video game are strictly against the rules and more than frowned upon by the developers who are prone to banning suspicious activity. Part-taking in use of these Modern Warfare 2 Infections is at your own risk. Not only is the warranty instantly in void the moment you open the case. Point being that if one of the rather delicate and easily broken parts malfunctions, you will no longer be eligible to trade in your busted machine. (This goes for anyone trying Black Ops based Infections as well.) Ye hath been warned!
What Sort of Infections Are There in Modern Warfare 2?
Modern Warfare 2 Infections run the gambit from interesting to overpowered, to down right game breaking. Common infections are unlimited ammo, instant reload, fast sprint, and "10th prestige." These minor enhancements allow you reload nearly instantly from a never-ending supply while sprinting two to four (or more) times faster than normal. The 10th prestige infection lobbies are possibly the most initially popular as they take the hours upon hours necessary to repeatedly grind up to maximum level and quickly grant you the highest possible prestige and all the titles and icons associated with it.
Over Powered and Beyond
Over powered Modern Warfare 2 infections up the ante and begin the make playing the game pointless. Two such examples include using a wallhack and Chopper gunner ammo infections. A Wallhack infection grants you the ability to see enemies constantly surrounded within a square or block that follows enemy movement as they run around the level, behind cover or in the open, almost identically to the Chopper Gunner killstreak reward. Chopper gunner infections also exist, in this case the infection replaces all of your ammo with bullets the Chopper Gunners fire which explode on contact. Instead of needing to hit an enemy directly, you can shoot at their feet and the resulting explosions do the work for you.
The most game breaking Modern Warfare 2 infections completely ruin the experience for all sides as the nullify all skill required to play. One such example is aimbot, also known as "instant aim" and "auto-aim." This infection automatically forces the player to aim at the nearest enemy, sometimes even firing for them, resulting in nearly perfect accuracy. These infections are the most heinous hacks available and is subject to major pushes from hackers to work on public and especially rated games. There is a constant tug-of-war between the hackers and the developers with patches popping up to render one method of cheating impossible, but as the patches continue, one can see new methods are found all the time.