If any of those, (3) is the red herring. Your genetic markers are not write-once-at-gamete-recombination / read-only-at-every-transcription. The genetic code humans have includes many graceful degradation mechanisms that allow for plasticity in widely varying conditions. Many 'code sequences' are actually selected by the environment throughout an individual organism's life and the resulting signaling pathways can amplify or suppress various fallback modes.

There's a lot of these scenarios that are well documented, anything from the loosely defined and often poorly understood epigenetics processes, but also the signaling regulation of cell growth and death, resulting changes with hormesis, acclimatization, availability/scarcity of nutrients, hormones, pharmaceuticals, viruses, and even mental state.

(1) is typically the largest factor of all that listed, with experiments showing repeatedly that 'genetic markers' specifying risk for heart disease, cancer, etc all vastly improve with a reduction of excess body fat and elimination of recreational drug usage such as excessive alcohol and cigarettes.

And as for (4), definitely a trump card is not taking a dump in your drinking water supply without reasonable recycling treatment. Many people tend to associate the medical industry as responsible for the supposed improved living conditions of modern developed countries. However, if you look at the repeated succession stages of colonialization throughout the past several hundred years, it's obvious that when people in underdeveloped countries stop shitting in their food and water, illness and resulting death plummet until a few generations later when economically enforced diets and 'sensible medical care' are in full effect leading to a rebound of misery.