For the OP,
Not totally unlike the operation of a modern vehicular engine, the Canon body and lens operate in a closed loop manner. Both have processors, and those processors have a data bus that joins them, and.... importantly... its not really a finite system, but more of a fuzzy logic system.
You can have issues with teh camera, the lens, or both.
Here's a weird one - I got an EOS 7D, and it had issues. Additionally, I already owned a 28/2.8 that become a poison lens for the 7D. Most lenses would focus ok, not great, but ok. Mount the 28/2.8 and thenafter no lens would focus correctly...not even remotely so, except at about 4-6 meters, give or take a little.
The 28/2.8 had no issues on any other EOS body - EOS 5's (film), EOS 5DmkII, EOS 400.... all worked ok with this lens. The 7D had a real issue with THAT particular lens.
I sent the camera to Canon... they changed lots of stuff (so they said). It came back working fine. And the 28/2.8 is no longer a poison lens. Hell if I can say why.