I'd tell him there is evidence that the virus exists, and under the right conditions and proper scientific equipment you can see a virus.
However, there is no actual evidence of the existence of god, and you can't see him no matter what you do.
Message to E.W. Jackson, A VIRUS may be invisible to the NAKED EYE, but it CAN be seen through a Microscope whereas Sky Daddy has NEVER been seen even by using an Electron-Microscope, hence that is why Sane, Rational, Logical Atheists reject your Imaginary, Invisible Sky Daddy but dread things like viruses.
Another fucking idiot. We can see viruses, just not with the unassisted eye. Maybe if "god" showed up way out in space and Hubble got a photo some would believe. Personally, I might accept a verifiable photo a proof of extraterrestrial life, but still not god.