Water grids are designed to flow in only one direction; a drop in pressure allows potentially contaminated water to flow backward into the pipes, e.g., from a crack that allows a small amount of water out into the surrounding soil.

I feel like no matter the pressure, if there is a crack and a dirty puddle outside then it will always contaminate the water

No, if pressure is higher on the inside it'll just constantly go from inside the pipe to outside it, and outside contaminants won't leak in.

Similar concept... clean rooms are positively pressured to keep germs, dirt, etc. from entering the room.

Isolation rooms are negatively pressured to keep bad stuff from leaving.

ooooo....

in new delhi, india back 7-9 years ago i was living in a pretty garbage area by my normal standards and one of the things we had to be careful about was when to use the water pump. do it during off time and the water from the tap would be green, it would stink and taste like vomit. that was a really really terrible experience and bad living standards. unless you are poor and cannot afford any other reasonably okay living place, one should not be living in those conditions.

> unless you are poor and cannot afford any other reasonably okay living place, one should not be living in those conditions.

??? Are you actually saying poor people do not deserve clean water?

no.. absolutely not. i have "actually lived" there so its not like that.

Come on, dude: he's clearly saying, "Yes, (of course, obviously) really poor people in the world we actually live in have to face dirty water and worse, but this is not acceptable."