In the Michael Landon series, Little House on the Prairie, daughter Mary loses her eyesight. Can anyone remember what happened to cause it?

scarlet fever. I checked on YouTube and she never marries Adam. she lived with her parents until they died, and moved in with Carrie, her youngest sister until Mary died. 

Measles or something like that.

Yes, I remember. Scarlet fever caused it. She'd gotten scarlet fever when she was younger. The extremely high fever caused damage that over time led to the loss of her eyesight. It sometimes happened that children who got scarlet fever would eventually lose their eyesight, and that's what happened in real life to Mary, the sister of Laura Ingalls Wilder.