[Fomite]

You can indeed do a proper clinical trial. Effect size matters. Something that provides a complete cure will have a commensurately big effect size, especially since in this case, you have pretty clear markers of cure.

An RCT comparing fecal transplant vs. vancomycin for recurrent C. difficile infection, for example, was halted after recruiting only 43 patients because it was judged no longer ethical to keep people in the vancomycin arm.