This is just a simple example of how bathing/changing clothes can affect your health. In the middle ages, you would have been walking in pathogen soup in all those dirty, sweat/urine/etc laden clothes you couldn't change out of.
Don't discount the impact of simple cleanliness. Pristine countryside? Doesn't exist. Go travel to one of those beautiful vistas you see in paintings, untouched by humanity and drink the clear stream water. Then spend the next few days doubled over in pain from all the parasites that live in the water, having arrived there from animal feces.
Nah, in the middle ages you'll be drinking whatever water is in your village's stream or watering hole, and you'll be glad for it too, after you carry it half a mile on your back to your house. If that water gets contaminated with cholera, you'll blame the sickness on the wrath of an angry god, or perhaps a jealous neighbor who witched you.
> Bathing [...] is not that important for health.
What???