The combination of cooking our food, and having access to clean water, and having waste disposed of in a sanitary manner does wonders for cutting down on parasites and disease.
Even so, there are probably many more parasites in/on you then you realize. Most of them are relatively harmless and difficult to notice, like those mites that live in most people's eyelash follicles.
Just for fun, give these a google image search:
Anyways, yeah, having good water conditions does wonders, but also just having some control over the tank and food cuts out a lot of parasite vectors.