What is the expansion of word virus?

The word "virus" doesn't have a formal expansion like an acronym. It's a Latin word that means "poison" or "venom."

The term was first applied to infectious agents in the 19th century, as scientists recognized that these tiny particles could cause disease, much like a poison would.

While there isn't a specific expansion, you might think of it as a shortened form of "virulent us" (referring to its harmful nature and its small size).