Wis. Stat. § 448.12: Malpractice.

Anyone practicing medicine, surgery, osteopathy, or any other form or system of treating the sick without having a valid license shall be liable to the penalties and liabilities for malpractice; and ignorance shall not lessen such liability for failing to perform or for negligently or unskillfully performing or attempting to perform any duty assumed, and which is ordinarily performed by authorized practitioners.