You're not understanding. The virus literally states that it became Mercer. The virions inside the vial took over his body, infected a few cells which in turn facilitated a rapid chain consumption of all the cells in his body.
"We don't know that was a single virion. We see one, but the dialogue doesn't seem to imply that."
We do know that because that is what is shown. The burden of proof would be on you to prove that there's more than the one virion we do see. For reference, Mercer changed Heller's entire biology in seconds with his hand.