I don't get it. Both sound right.
Like, take for example COVID. If we were all Amerimutts, we'd be falling dead like flies.
Their diet is shit and they're all fat as fuck with weakened immune systems, but at the same time, what makes someone sick is the virus infecting the cells. You cannot make an entire population lose weight, and live a healthy lifestyle, so germs cause disease and a weakened immune system increases the probability of these germs infecting the these cells.

This can apply also to things like cancer. The reason why young people don't get as much cancer as old people is because our immune system catches those cancer cells early and kills them off. However, as we age, our immune system weakens and mutations increase, so methods that would toughen up the immune system would not be as efficient anymore.

In other words, it looks like a false dichotomy more than anything, so why is this even a debate?