Blindness due to childhood illness

My character is growing up in a medieval/renaissance society and is blind due to an illness in childhood. It's important that the blindness is clearly not congenital (i.e. nobody is going to shy away from marrying her for fear she might produce blind children), that she has once been sighted and thus has the concept of sight, colour, etc; and that she is not otherwise disabled or disfigured.

I've tried Googling "blindness+childhood illness" from which I gather that smallpox, scarlet fever, meningitis, measles and mumps can all sometimes cause blindness. I think smallpox is a non-runner because, as I understand it, smallpox is normally highly disfiguring as well - I don't want her to have any visible disfigurement or any other deficits. Can anybody advise me which of the other is most likely to simply leave a child blind, without any other complications?

Another question: what would be the optimum age for a child to retain the memory of sight and the concept of all that sight entails, but still be able to adapt fully to blindness, using echolocation etc?