Scientists ought to apply universally accepted definitions to technical
terms to enable precise communication and discussion. Throughout
history, many definitions have shifted, such as those of important terms
in evolutionary biology, modes of life, and the nature of interactions.
Conversely, throughout history, important terms have been used without
proper definition. Two such terms are parasites and pathogens. An
extensive body of literature considers parasites and pathogens to be
categorically negative, which has resulted in definitions that lack
objectivity and limit a full understanding of the biology of these
organisms. Different interpretations have resulted in shifting
definitions, in turn causing confusion. Here, we present the remarkable
history of both definitions, an overview of alternative definitions put
forward throughout history, and a working definition for both terms. We
find that the line between what is a parasite or a pathogen is often
blurry, and is additionally complicated due to the multi-modal nature of
interactions.