I guess the difference here is that you are assuming they are drinking excrement filled water, where I was thinking they are drinking fresh water. There is probably a large difference between those that began to live in more crowded cities and those that were still in more pristine country side. It is not clear cut whether people were smaller in the middle ages.
http://researchnews.osu.edu/archive/medimen.htm
Bathing or changing clothes is really a social aspect that is not that important for health.
I live in the USA on a small farm in an ordinary, modern house . Yesterday I removed two ticks from my body. In a few weeks, I'll be doing it daily, and checking my son for ticks at every change of clothing (he knows to kill the ones he finds, but he can't check his own back). We mostly have wood ticks, but there can be deer ticks along with them. Deer ticks carry Lyme Disease.
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.
The odds of a healthy individual becoming ill from drinking water in pristine wilderness is actually extremely low. There is really no evidence for your assertion that you will automatically become ill from drinking unfiltered water once. My understanding is there is little risk if the water is not under upstream human or livestock use.
http://www.wemjournal.org/wmsonline/?request=get-document...
Very few people in the “middle ages” lived in pristine wilderness. The vast majority were, and always have been, rural peasants in feudal or semi-feudal societies, working as serfs or sharecroppers or slaves, either on tiny individual plots, or on plantations. They became ill constantly, and most children died before age 5. You seem to be imagining some kind of hunter-gatherer society.
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.
