Definitions of parasites and pathogens through time

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.