There are over thirteen eye diseases ranging from the common, easily treated and not too serious Dry Eye condition to the very rare Neurotrophic Keratitis, whose treatment is very expensive indeed. You will have heard of mascular degeneration, common in the elderly; glaucoma and cataracts.
The latter can be treated easily and painlessly these days. Retinal Detachment is serious and requires surgery. Blindness is a condition with different causes rather than a disease.