Remarkably KI is gentle on the body while still doing a great job purifying water of the most common foul bacteria and fungus.

This seems like it would make no sense until you stop to consider that idea that in earlier generations of man we ate on average a lot more marine plants than we do today (kelp in particular) which are rich in Potassium Iodide. People ate kelp because it was a "low hanging fruit" so to speak, easily harvested in great quantity from the sea. Perhaps the beneficial organisms in our gut evolved to tolerate KI so that populations could tolerate eating mostly seaweed.

The Japanese still eat a lot of kelp (the average japanese male ingests more than 40mg of KI daily from seaweed), and if you crunch the statistics it looks like they may have significantly less AD among their population than we do in the west: http://www.worldlifeexpectancy.com/cause-of-death/alzheimers...

Not everybody lived nearby a sea. My family is from the north west of Argentina (Jujuy). To reach the sea you have to travel a few hundred kilometers horizontally and more than one kilometer vertically, so bringing anything from the sea was very expensive.

Moreover, the iodine content of the food was very low and that's bad for your thyroid. The government mandate to add a small amount of KI to the table salt NaCl. https://en.wikipedia.org/wiki/Iodine_deficiency