Hello,
The only things that are infections are DVAR's.
In order to get yourself infection you will need someone to have a JTAG and Host a lobby with the edited Patch_MP file.
Once you enter the lobby, it will change your DVAR's for multiplayer.
When you go back to the main menu and/or turn off ur xbox your DVAR's will return to their normal values.
So,
If you get infected, make sure to leave your Xbox on with a FAN blowing onto it, and as long as it is on and at the multiplayer/systemlink menu you will be infected.