If you visit a developing country, you'll still see people bathing and washing clothes and cookware in rivers. You'll also see outhouses on stilts along the same river. While there's a high incidence of illness among people who live like this, many people clearly tolerate it well. If I were to do that now, I'd get sick, but eventually I'd probably be able to tolerate it as well (assuming I survived).

It's a common misconception that animals don't get sick from tainted meat or water. They do, and when they're weakened enough, they get eaten. Clearly some have a higher tolerance of certain pathogens (e.g. vultures), but many succumb to intestinal bacteria and parasites at alarming rates. We just don't see it.