They are just sick like all the damn time.

It’s a misconception that wild animals don’t get sick drinking dirty water. Humans can drink dirty water, they just get really sick - animals do too. Some have microbiomes that do offer some protection, but, animals get sick from dirty water all the time.

wild animals get sick all the time, and this includes waterborne diseases. some of them are packed with nematodes, for example

We’ve evolved to spend more energy on our brain and intelligence. Animals can have better immune systems, plus they’ll build up a resistance to the pathogens.

People who constantly drink dirty water will also develop resistance

I think they basically have built up a tolerance to those bacteria. They probably got sick at some point and survived.