Autism is a spectrum disorder. That is, there are autistic people who barely function; then there are those who are "high functioning" - and can manage an independent life. (At least) some high functioning autistics demonstrate superior calculating or memory skills. In previous posts on the subject, I've illustrated how the brain can just... well... be wired differently.
NPR reports on the fever effect: some (not all) autistic children that may be uncontrollable, prone to tantrums and self-harm most of the time, can demonstrate relative lucidity when they have a fever.
Interesting, when you consider that "feverish" people are traditionally seen to demonstrate less lucidity. Again, there are a number of possible suggestions about how autism affects the brain, how fever affects the brain, and how the two interact. This demonstrates how much we have yet to learn about brain functioning.
The full article in the link above gives useful detail on this fever effect.