The parent of one of my kindergartners is a micro-biologist. She recently prepared and taught a lesson for us on the importance of hand-washing. Each child was permitted to touch a sterile substance in a petri dish. They labeled the dish with their name and the dish was sealed shut. Surfaces around the classroom were swabbed and treated the same way. We observed the dishes over a few days and it was interesting to see the variety of growths that developed.
These were known fungi that she brought for demonstration purposes. But she also identified bacteria that tend to not appear as fuzzy as the fungi.
Because of the children's names, I chose not to photograph their dishes. They varied some, as would be expected. These dishes below show some of the growths from the various surfaces.
(Computer mouse, not varmint.)
By far, the most active growth came from the shoe bottoms.
So the lesson is, wash your hands and keep them out of your mouth.