Sources of beneficial and disease-causing bacteria?

Beneficial and disease-causing bacteria are living organisms that belong to prokaryotes. They don’t appear on their own. Any non-sterile surface and even an organism is a carrier of billions of bacteria. The most common source of pathogenic bacteria are fragments of corpses, places with a good climate and the presence of a nutrient medium. A nutrient medium is also necessary for beneficial bacteria. But clear sources of beneficial and pathogenic bacteria cannot be identified, since they live almost everywhere and are not separated from those dangerous and useful.