"The term was used in late Middle English to describe snake venom and later evolved to refer to the substances within the body that caused infectious diseases.

The connection to poison was a natural fit for early descriptions of disease-causing agents, which were seen as harmful and destructive, a concept that persisted when the term was later applied to the microscopic infectious agents discovered in the 19th century."

Yeah, I mean all that kind of tracks. Virus, poison, these aren't that far away from one another and when something new is discovered we apply a name to it.