To me autism is defined by how some of our genes work(or doesn't work). I'm not into analogies, but the difference between normal Aspie and nonverbal person is not that big - by using your analogy both of them have stomach pain, but the result seems to be wildly different - one of them is able to communicate almost like other normal humans and they can't even tell that Aspie has a stomach pain.
As for different medical conditions - imagine, that previously mentioned Aspie in different environment can have stomach pain, because it is caused by environment and not by stomach pain. Yes - there are some medical conditions, that also comes from genetical conditions and those just happen to be in neighbourhood to autism for no other reason than just general damage to DNA. And the issue here is that we actually do not fully know how DNA functions - only some of the bits.
As for different medical conditions - imagine, that previously mentioned Aspie in different environment can have stomach pain, because it is caused by environment and not by stomach pain. Yes - there are some medical conditions, that also comes from genetical conditions and those just happen to be in neighbourhood to autism for no other reason than just general damage to DNA. And the issue here is that we actually do not fully know how DNA functions - only some of the bits.