India, Nov. 17 -- When he stepped into his old friend's apartment earlier this year, the stench hit him before the sight did. The floor was strewn with garbage and human waste, insects crawled around freely, and his college mate, once a first-class commerce graduate preparing for a chartered accountancy career, now looked gaunt, disoriented, and lost inside a world of delusion. He spoke fluently about Donald Trump and Vladimir Putin visiting him soon but could not explain why his home was collapsing around him. For the friend, who had not seen him in decades, that moment changed everything.

"He was speaking as if he was a different person," he recalled. "But there were also times when he remembered our college days clearly. That made it ...