Polish wikipedia says up to 86% of seeds carry it but without source. This paper  says 70% of 'wild populations' of related L. perenne, implies infection in darnel (L. temulentum) is also prevalent, but notes most populations have low frequency of infected individuals and then tries to explain that.
p34 talks about its constituent properties. Crucially no one is quite sure what the exact active component is
It's suspected that ergot caused the Strasbourg dancing plague of 1518