Are you familiar with c diff? Placebo's don't work. It's dangerous to have placebo controlled trials with this disease. The current standard of care is a course of very strong antibiotics which still leave > 25% chance of recurrence. These antibiotics are most often used as a control group in c diff infections.

In general, placebo controlled trials are usually conducted for diseases which don't have an effective standard of care, or for conditions which aren't serious enough to be considered life threatening. For more serious diseases where a standard of care exists (standard of care is usually scientifically discovered but validated by placebo controlled trials), you conduct trials and compare the performance of the new drug/therapy with the existing standard of care as the control group.

"The benefits, risks, burdens, and effectiveness of the new methods should be tested against those of the best current prophylactic, diagnostic, and therapeutic methods. This does not exclude the use of placebo or no treatment in studies where no proven prophylactic, or therapeutic methods exist." World Medical Association (WMA) Declaration of Helsinki.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2647647/ for more.

Placebo controlled trials for C Diff would be unethical thus would likely not be legal in any country that conduct their trials with any ethical standards. Just like you wouldn't have a placebo group in a trial for an new AIDS or cancer treatment. In this case we're looking for a treatment that works just as well or better than current treatments.