For example: how do you avoid drinking alcohol if your only sources of drinking water are contaminated? You can boil the water — but you don't know a priori whether the pathogens or toxins in the water can be destroyed by boiling, or how much boiling is needed. If the toxins are things like arsenic, boiling the water will concentrate them, while various kinds of biological processing may filter them out.

And where do you get vitamin C? Tomatoes are a great source of vitamin C, but you can't just go around eating random local tomato-like plants; almost all of them are deadly. Plants like rhubarb have lots of vitamin C, but also lots of oxalic acid. Raw meat is also a good source of vitamin C, but raw meat can harbor many different pathogens. Are you going to build a microscope, synthesize some stains, and inspect a bunch of meat samples to see which ones bear pathogenic bacteria?

And once a couple of your human-subjects experiments go wrong, or just have coincidental unrelated deaths, you're a candidate for a witch-burning.

(Even in the best cases, modern hygiene is not without its drawbacks; the poliomyelitis epidemic is one example.)