most problems (especially those not resolved with diet and exercised) are caused by chronic infections. many cancers, likely alzheimer's, event heart attacks (look it up!).

but ultimately that's an immune system issue. we are exposed to pathogens throughout our lives. as we *age*, our immune system weakens then it all starts killing us. sure some people are born with weakened systems, and others were exposed to chemical damage. but most of us simply accumulate damage to the immune system as we age.

you can't fix that by "being exposed to less pathogens" or "training your immune system in advance". (masks, vaccines)

you can't fix it with diet and exercise. (it helps, but it's not a cure)

those are nice short-term solutions

the correct fix is to "rejuvenate the immune system"

that's the only long term and permanent solution