In the 1690s, Salem was home to a number of Megans, who lived in the city under the guise of ordinary people. When some Megans made mistakes and accidentally used their powers, a number of them were executed as witches by the Humans living in Salem. The surviving Megans left Earth and returned home, fearful and bitter towards humans for the next several hundred years because of their experiences. (TAS episode: "The Magicks of Megas-Tu").
Although most of those using magic at the time had indeed been Megans, some were Humans, temporarily driven toward wild actions from tainted fungi (ergot) in the locally-produced bread. (VOY novel: Cybersong)