I heard the statement a couple of times that places in this world with more cleanliness tend to have more auto-immune illnesses such as allergies. One can imagine that the immune system also needs to "exercise" from time to time, just like the muscles in our body or the brain.
What if "exercise" is a more general theme? Perhaps every pathway in a living system needs some form of exercise (but not too much, of course). What if the error-checking mechanisms and all the other anti-cancer mechanisms in our body also need exercise... perhaps we can cure cancer by exercising anti-cancer pathways. Exercise against cancer would be very tricky. It would require producing small controllable (or self-limiting) tumors in our body from time to time. By doing so, the hypothesis is that the body's anti-cancer pathways would remain alert (well exercised). This might make the body better prepared against the real cancer or tumor.
The same theme of exercise can perhaps be applied to achieve other goals, e.g. directed evolution of microbes.