Placebos have been shown to be quite effective in treating skin warts, which are clearly not a subjective ailment and are caused by viruses. According to an Australian physician, F.E. Anderson, warts probably have the highest number of folk remedies of any disease, which is not surprising if they respond well to placebos.
From the excellent Eight Preposterous Propositions, by Robert Ehrlich.
Francis Bacon recommended treating warts with pig fat. Sometimes warts respond well to hypnotic suggestion, an effect which is not well understood. And this is from an author strongly opposed to pseudo-science.