Ok, so it's not really a theory, but it's by far the best explanation I have ever heard of what it's like to live with a chronic debilitating illness. Since is a copyrighted piece, I can't copy and paste it, but it's a must read. For whatever reason, my lovely iPad isn't allowing me to hyperlink, so please just copy and paste into your browser. It is very worth a read.
http://www.butyoudontlooksick.com/articles/written-by-christine/the-spoon-theory-written-by-christine-
-MS