There have been some amazing scientific findings regarding stress and the human body.
Several of my friends suffer from or, thankfully, have suffered various painful conditions. I recently underwent a very painful time in life regarding my back and the ability to walk for which I am still recovering. I mention this because the above article talks about a theory of how to explain to someone how one lives with Lupus but I feel it is applicable to all sorts of chronic or crippling conditions.
So take some time to read it and think about your friends who have to control their conditions. Hopefully it’ll make you a better friend. :)