As a small child my IQ was apparently off the charts high - there was a lady hired by the county whose job was to go around testing the intelligence of all the 18~ month olds in the county. Most kids, when shown a picture of a dog, would say "doggie" or "woof woof." When I was shown the picture, I was able to correctly identify the breed - I said, "Collie!" The lady said I was one of the smartest kids she ever tested (she was quite disappointed a few years later when she came back to test my little brother - she had high hopes for him, but he tested totally average, saying "woof woof" when the collie picture came up). I'm not sure if you can quantify a toddler as being a "genius", but I definitely tested far higher than average on the intelligence test I was given at that age.
I think I've gotten far stupider with age/illness though.
My memory is terrible and sometimes I just feel rather slow mentally, particularly when my symptoms are hitting me hard. Oh, and the funny part is, now that we're adults, my brother tests at a slightly higher IQ than I do (neither of us are geniuses though!) - I guess he caught up to me and surpassed me, or I have gotten dumbed down to the point where he's now smarter than I am.
(He has no chronic illnesses FWIW.) We both test above-average nowadays, so I'm still smarter than the average bear I guess, but I'm no longer the smarty-pants I was when I was much younger.