Cause unspecified. Nazareth formed in Dunfermline, Scotland in 1968 and hit it big in the USA with their 1975 album, Hair of the Dog and their cover of the Everly Brothers' Love Hurts.
https://www.rollingstone.com/music/music-news/dan-mccafferty-nazareth-singer-dead-obituary-1234627327/
https://www.rollingstone.com/music/music-news/dan-mccafferty-nazareth-singer-dead-obituary-1234627327/