You have got it backwards. The disease is the human body not being able to do optimally with some environmental and/or behavioral pattern. Upto this point we didn't have much choice but the change the environment or behavior. But nothing indicates we cannot just fix ourselves to handle the environment better.

Take allergies, manageable by adjusting your environment so you never come into contact with the allergen. It would be much better to cure you allergies however. And I don't see how this is a physical impossibility.