During the civil war, how many people had subsisted their entire lives on water from a municipal treatment facility, nearly 0%? In the USA right now, today, it's EASILY less than 5% of the population that's strictly on their own private well and has been their entire life. Our digestive and immuno-biology is fundamentally changed from 160 years ago. And again, in a total breakdown, water-treatment and sanitation, even indoor-plumbing becomes non-existent.
Think US tourists in Mexico sans bottled water, it'll be that way EVERYWHERE. You get a stomach bug, you get the runs, you get dehydrated, your kidneys shut down and you die in under 2wks. There's no anti-biotics to bring you through and your body isn't equipped to do it for itself.
On the Oregon trail, the biggest killer by far was disease, usually Cholera or typhoid. They're spread by polluted water. Everyone drank from and washed in the rivers. Sick people would wash off puke and diarrhea in the rivers and spread it to everyone drinking down stream. Drinking from a home well is fine as long as you have a working pump so when the grid goes down, you need a power source of some kind. A lot of people will end up drinking from streams.
As general interest, the 2d most common cause of deaths on the Trail was gun accidents. All of the immigrants had guns but a good share of them had no experience using them. They were farmers who had never hunted. They ended up shooting themselves and others accidentally.
