I've always wondered why placebo works for minor problems (a cold, nasal congestion, mild pain) but it does not for real diseases. For example, you never see ch

I've always wondered why placebo works for minor problems (a cold, nasal congestion, mild pain) but it does not for real diseases. For example, you never see chemotherapy treatments compared to placebo (because it is unethical to conduct such a study), but it could be done with "wait and monitor" cancers such as slow-growing prostate cancer.