You’re right that viruses don’t “do” anything on their own, they get our cells to do things for them. But calling a virus a “chemical” (which usually implies a substance composed of a single type of molecule) is also overly simplistic. The genome of viruses can be quite sophisticated and can instruct our cells to do lots of things that are to the virus’s benefit, like making certain chemical messages that interfere with the functioning of our immune systems, helping the virus to evade detection.

A prion is closer to what you’re describing, consisting of truly just a single protein.