Show that microorganisms don't cause disease? Or demonstrate that the majority of diseases like cancer are not caused by microorganisms. Maybe that the specific pathogen is simply opportunistic and exploits already diseased cells?
Don't misunderstand what I am trying to say. I am not disputing Germ theory, Evolution, Relativity, QM, or even Newton (I use F=MA every day). All I am trying to say is that science is a process of falsification and elimination, not proof.
You would have to show that no disease is caused by microorganisms.