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The terms pathogenic and non-pathogenic are often are applied to various microbes. By definition, a pathogen is a specific cause of a disease, while a non-pathogen is considered harmless. In reality, the distinction is not always clear. In 1890, the German physician Robert Koch formalized the criteria to classify bacteria as pathogenic. (See Koch's Postulates)
While these definitions made sense at the time, advances in microbial sampling and identification have shown that they are unable to account for microbes that cause disease in some individuals but are also present in normal individuals without causing disease. A harmless microbe could become an opportunistic pathogen, especially in an immune-compromised host. For example, Staphylococcus aureus, a bacterium that is part of the normal human skin flora is also a common cause of nosocomial (hospital-acquired) infections.
As a formulator is often tasked with utilizing ingredients to prevent the formation of pathogens on the body while preserving beneficial skin non-pathogens, understanding the two is essential.