I worked in health care for about 17 years and noticed that some people are always sick and some never get sick, some have chronic pain with no apparent cause and some have no pain when you would expect them to. I always felt that there was something more to it than biology and I started reading everything I could get my hands on and in early 1997 I became very interested in metaphysics and my conclusions are similar to what Steve posted. I believe that we are triune beings with a mind body and spirit and that illness can attack us at any of these levels.
When we get sick on a spiritual level I believe the cure must occur on the same level. I think this partially explains the high relapse rate in some cancers, like when someone undergoes all the latest treatments, start getting good checkups and maybe even a "clean" status , then out of the blue, they are back at stage 3 or 4. Of course medical science has their own explanations for this but I feel they are just inventing reasons because the answer lies outside their realm. They typically will just start spouting statistics as if they were causes.
I think we can affect the human body from any of these levels, the most common being the physical 'cure' and if the problem hasn't affected the other levels of being it will be sufficient but, the problems like cancers, addictions, and other chronic illnesses I believe are best treated at the highest level and that lower level treatments only relieve symptoms and provide a temporary reprieve.
As for who caused what, I have a suspicion that collectively we all may contribute to some of these diseases. I feel that if we aren't properly using our creative faculties, meaning pro-actively, that we may fall prey to what earlier writers called the "mass mind" and be influenced by it, and guess what, in the "mass mind" there is a huge belief system in place that owes it entire being to illness,it's called modern medicine. There are lots of people banking (literally) on people being sick, and I think they influence the "mass mind" in a way that perpetuates sickness, not purposely, but out of ignorance of the LOA. They count on people to be sick in order to make a living. I'm not saying it's a conspiracy or something, just misuse of our faculties.
A friend of mine who is an oral surgeon and makes about 1M a year told me, when I asked him why we have wisdom teeth if they usually end up needing to be pulled, "I always figured it was so I could afford my Bentley".
. Of course he was joking, but I think there is maybe more truth there than he realizes.