Q&A: What is a pathogen? A question that begs the point

A pathogen is usually defined as a microorganism that causes, or can cause, disease. We have defined a pathogen as a microbe that can cause damage in a host. However, this definition immediately raises the question of what it is about the microorganism that enables it to cause disease or damage; and this takes us to an ongoing debate that dates back to the late 19th century when the germ theory of disease was established. In the early days of the germ theory era many of the major pathogenic microbes were encapsulated or toxigenic bacteria, and this suggested that there were inherent differences between pathogenic and non-pathogenic microbes. However, even then it was obvious that neat classifications were problematic, for it was known that a microbe could be attenuated in the laboratory, but virulence could be restored by passage in a host, suggesting that the same microbe could exist in pathogenic and non-pathogenic states.