There was once a study to test the amount of "poop particles" (feces based bacteria) on everyday objects. The study consisted of putting objects in places that would be more or less likely to have feces and a control group which was isolated from any source of feces based bacteria to the best of their ability. The microbiologists running the study were unable to tell which group was the control.

This is written to the best of my memory and some details may be wrong but the meaning is the same