Ok, so this is a true story.
I live in Vermont and most people get their water from wells. We, however, have a spring that is gravity fed.
Last year, it smelled a bit odd (us and three kids were drinking it) and I went up and took off the concrete cover. Low and behold there were three dead snakes in the water and a dead rat (I think). All floating and bumping against the intake line that went to the house.
Cleaned it out, shocked it, and then a couple of months later , the pump it he house stopped working. When we took it apart, there was snake bits chewed up in the impeller. It had slithered through the pipe 200 yards to the house.
All still alive, have healthy immune systems, but funnily enough, don't drink the water from the boat tank
My wife would sh!t her pants if she read this story (a bit of a problem with snakes)!
Back to the OP. Municipal water pipes are often really old, and never cleaned. They don't cause (too many) problems because there is a constant flow of clean water through them.