It does not need to be actual "mental illness". We need to understand that even healthy, reasonable, adult minds are subject to delusions and hallucinations. We are just barely beginning to understand human neurology, but what we are starting to learn tells us that we cannot depend on our own senses. And yes even groups of people, healthy people, can suffer from shared delusions. Especially when properly primed and encouraged. It is so amazingly easy to deceive ourselves. I place absolutely no trust in personal testimony, even from groups of sober adults, it has been proven unreliable over and over again.