in fact, they actually do do this, don't they? Quite a lot of COVID-19 deniers demanded proof of "isolation" i.e. they demanded someone to show them a vial of pure COVID-19 virus and then somehow prove that's really what's in the vial. And they demanded proof of "Koch's postulates" i.e. that injecting people with the stuff in the vial gives them COVID-19, etc etc.

e.g. this FOIA request: https://www.ons.gov.uk/aboutus/transparencyandgovernance/fre...