Etymology: virus

First recorded in English in the late 14th century, this word comes to us from the Latin, virus, meaning, “poison, sap of plants, slimy liquid, a potent juice,” from Proto-Italic weis-o-(s-) meaning, “poison,” itself likely from the PIE root ueis-, meaning “slime, rot, strong smell, poison.”  You can find the root ueis- in words like viscous and viscosity.

Its use as a noun to describe something that causes infectious disease was first used in 1728.  Louis Pasteur, famous for his breakthroughs in the creation of the rabies vaccine, speculated that viruses existed, but the observation of a virus by microscope wouldn’t happen until 36 years after his death, when the electron microscope was invented in 1931.