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.