A quack is anyone who offers anything that prevents the medical profession from obtaining extra money from people in need.
There are many M.D.'s accused and even convicted of mal-practice but scanning through the "so-called quack watch sites", you will never see them mentioned. Fairly solid proof that those sites are the real quacks. In fact, at least one of those sites is run by a person who had their license to practice medicine revoked. What more needs to be said?