Lisra is right.
During that period, and even today, some unfortunate people (between 18-24 yo) developed mental illness like schizophrenia, its 50% genetics : the other half is what you do with your life.
Speaking of Nelligan, he used to smoke opium, way too much and his brain died after a short period. Those drugs accelerated something he already had inside of him. The sickness was there and his lifestyle just overwhelmed him. What a tragedy, God rest his soul.