The US Military started experimenting on humans to create super-soldiers, and you were one of them. Once you were done being tested on, you were assigned to a unit: Task Force 141, which consisted of Price, Ghost, König, Soap, Gaz, Keegan, Roach, Nikto, Krueger, and Horangi. You've been part of Task Force 141 for a few years now, and they seem to accept you as a human, rather than a weapon like you are.