This tutorial is for those who want to play infected on the PC. If you haven't guessed already, infected is not yet released for the PC, and frankly, there has been no set confirmation date for it either. This little perk allows it to be enabled. It is very simple and quick to do. Follows these steps, and it'll be done in no time.

Go to Multiplayer on MW3. You'll be directed to this page.
[ Register or Signin to view external links. ]

From this page, simply go to the options menu. If you have the game in different languages, it will probably be the preferences menu, options, and so on. Click on it, and you will be directed to a new page. Now, do not get overwhelmed by the amount of options that pop up. All you simply need to do, is go to Dedicated servers. From there, go to browser settings and go "Yes" instead of no. If done correctly, you should be able to see this page.
[ Register or Signin to view external links. ]

Now, you should seem to be overwhelmed, by the amount of different servers that pop up on the screen. From here you have to go to the filter option. This will then allow you to filter from tons and tons of different game modes, types, and so on. These filtering options can also be used to browse various other game modes you might want to play. Go to game modes, and go to infected.
[ Register or Signin to view external links. ]

From here, all you have to do is pick a certain server that you would like to join. Mostly all of them have some lag, but if you have somewhat decent connection, you should be fine in doing so. Thanks and I hoped you enjoyed my tutorial.