I think you're creating a false dichotomy between "a real disease" and "a limitation of the body." Even for diseases that entirely result from external sources,

I think you're creating a false dichotomy between "a real disease" and "a limitation of the body." Even for diseases that entirely result from external sources, our ability to survive them is a limitation of our body (e.g. some people will survive the same flu that other people die from). And cystic fibrosis is a genetic disorder, making it quite literally a limitation of your body as well as a real disease.