influence (n.)

Influence means the power to affect someone or something. It comes from the medieval Latin influentia, a flowing in, from influere, to flow in.

In the Middle Ages, influence was believed to be a fluid that streamed down from the stars into people and shaped their character and their fortune.

Influenza is the same word. It is simply the Italian for influence, and epidemics were once blamed on the stars. Flu is what is left of it.

He was under the influence.