No question is ever dumb. Viruses are different in that they have no single cell in their structure - they are parasites that do not themselves possess a cell.
In other words, viruses, rather than being cells that can only replicate upon entering another cell, viruses do not even possess a cell until they enter (infect) another cell.
I hope this is making sense - it is hard to explain. They are submicroscopic parasites that can replicate once they have invaded a cell - whereas other microscopic parasites are single cell structures to begin with.
I hope this helps!