A pathogen is any living thing or substance that can cause disease. The term was coined in the late 19th century before a full understanding of the differentiation of different types of organisms within germ theory. It would have included what are now known as bacteria, viruses, fungi, parasites and even toxins. As the etiology of diseases is now better understood, and includes new disease causing agents like prions, the term is only used in a non-technical sense to refer to organic disease causing agents, although the term lives on in the specialty of pathology.

Pathogen at Wikipedia

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