What is "geisslerlieder"

In medieval music, the Geisslerlieder, or Flagellant songs, were the songs of the wandering bands of flagellants, who overspread Europe during two periods of mass hysteria: the first during the middle of the 13th century, and the second during the Black Death in 1349. The music was simple, sung in the vernacular, often call-and-response, and closely related to folk song; indeed some of the flagellant songs survived into the 17th century as folk songs in Catholic parts of central Europe. Musically the Geisslerlied were related to the Laude spirituale: they were unaccompanied song, with instrumental accompaniment specifically forbidden.