That same night I washed myself and went into my courtyard and slept by the wall of the courtyard; and my face was uncovered because of the heat. I did not know that there were sparrows on the wall; their fresh droppings fell into my eyes and produced white films. I went to physicians to be healed, but the more they treated me with ointments the more my vision was obscured by the white films, until I became completely blind. For four years I remained unable to see.
—Tobit 2:9-10 (NRSV)

This is possibly the weirdest cause of blindness I've ever seen.