No explanation of the origins of blindness were given in the book. Quoting from this Wikipedia article:

Blindness is the story of an unexplained mass epidemic of blindness afflicting nearly everyone in an unnamed city, and the social breakdown that swiftly follows.

No explanation of the origins of blindness was given in the book. Quoting from this Wikipedia article:

Blindness is the story of an unexplained mass epidemic of blindness afflicting nearly everyone in an unnamed city, and the social breakdown that swiftly follows.