From BBC News:
He died of autoimmune inflammatory vasculitis, a rare disease that involves swelling of the blood vessels.
The star found fame as bespectacled ghost-hunter Egon Spengler in the Ghostbusters franchise in 1984. But he was also a talented writer and director, whose credits included Caddyshack and Animal House.
"His creativity, compassion, intelligence, humour and spirit will be missed by all who knew and loved him," said his family in a statement.
The star had reportedly been quiet about his illness, which dated back to 2010.