What was Nathaniel Ayers's illness in The Soloist?

The Soloist was published in 2009 and features the real-life story of Nathaniel Ayers. The author, Steve Lopez, was a columnist for the Los Angeles Times when he met Ayers on the streets of Los Angeles. When they meet, Ayers is playing a broken violin with two missing strings and claims to have attended The Juilliard School (referred to as Juilliard). Lopez can clearly see that Ayers is homeless and disturbed in his mind and is doubtful that the man attended one of the most prestigious music schools in the world. Lopez assumes his role as a journalist and begins to uncover the life of Nathaniel, and he finds out that the man had in fact attended Juilliard in the 1970s.
Nathaniel Ayers began his first year at Juilliard in the Fall of 1970 as a double bassist. Under enormous pressure to perform academically, he started exhibiting troubling psychological symptoms, and his grades declined. His mother noted a change in personality, paranoia, newfound religious obsessions, and confusion of thought and speech. In his second year, he experienced a psychotic break after consuming marijuana and was diagnosed with schizophrenia. After undergoing traumatic electroconvulsive shock therapy, Ayers ceased all treatment for his mental illness.
When Steve Lopez meets Ayers in the 2000s, Nathaniel has been living on the streets for decades. Due to years without psychological treatment, Nathan's symptoms have worsened significantly. He experiences frequent bouts of psychosis, including auditory and visual hallucinations. His moods vary unpredictably, his emotions rapidly shifting from happy to fearful to angry in his interactions with Steve. Over the course of two years, Steve attempts to convince Nathan to seek treatment and live a normal life. Ultimately, it becomes apparent to Steve that he must accept Nathan for who he is, and that his friend has adapted to living on the streets with a mental illness.