Back in 1981, Dr. Melvin Gravitz (a doctor at George Washington University Medical School) published an article in the American Journal of Clinical Hypnosis
in which he noted that it's possible not only to cure warts by hypnotic treatment (sometimes) but also to produce warts by suggestion as well.
Gravitz wasn't actually doing this himself, but he reported that back in the early 20th century (and presumably earlier as well) creating warts was a popular pasttime among girls in the Swiss canton of Vaud:
Charles Baudouin, a contemporary Swiss, noted that the canton or province of Vaud, in which Lausanne is located, was well known for its large number of lay wart healers. It was also possible for a patient to employ a prescription for the treatment of warts without consulting one of these folk healers. "In these cases, autosuggestion is seen in all its beauty. Prescriptions pass from village to village and
hamlet to hamlet. Some of them are incredibly quaint. For example, to cause
warts, the subject goes out one evening, moistens the tip of the finger, looks
at a star, and simultaneously applies the
wet finger-tip to the other hand. The operation is repeated, the finger being freshly moistened with saliva each time, while the subject counts, 'one, two, three . . .' up to the number of warts desired. Now, wherever the moistened finger-tip has been applied, a wart duly appears."
He noted that such practices were a form of amusement among the Vaudois girls who derived pleasure from passing their own warts on to someone else. "A ribbon is tied around the affected hand, and is knotted as many times as there are warts on the hand; then the ribbon is dropped on the highway. Whoever picks it up and unties the knots, will get the warts, and the original owner of the warts will be cured."