I had superb vision even for many years with ME/CFS though my eyes did tire easily. I had a virus when I was 40 and was in hospital for a couple of days. When I went to read the newspaper after I came home I couldn't see well enough to do so. I could read it by moving it an arms length away though. The optometrist I saw shortly after laughed at me when I said it happened suddenly that I couldn't read at a normal distance. He said it was because I was 40, that the changes happen slowly and that I must have been compensating by moving things further away to read for quite some time. I didn't bother to argue with him. For reading I went through 3 script changes in 2 years which surprised the optometrist. For the last few years my eyes tire to the point I can't focus properly after a short time reading. The last optometrist I saw actually understood that. When I mentioned the difficulty with focus he said that would be because of the ME/CFS that he saw in my notes, that the eyes are muscles and would be expected to fatigue quickly. Turns out his wife has a post viral fatigue syndrome so he is somewhat familiar.