I'm not entirely convinced that there was ever a time and place when you could step out into the woods and indescriminatly scarf down any food or water you kind. Giardia, otherwise known as beaver fever, which tels you one of the natural carriers, was widespread in wilderness areas. The real difference was that most humans back before the 1900s had imunity to giardia becuase it was ever present in their water.

Similarly, most people had some exposure to cholera and other common bugs that were common to untreated water. Limited exposure afforded tham a degree of resistance, if not outright immunity. Only when exposure exceeded incidental levels did cholera and many other diseases become active.

What people are missing is that many of the common diseases and germs are present in small amounts in soil or populations. Anthrax is endemic to much of the southwestern US. The difference between the natural occurence in soil and an ooutbreak is degree of exposure and readiness of the germs to be infectious because they somehow gained the upper hand in the environnment.