This is one of the stranger medical correlations I have heard of.

There is evidence that running a fever actually allows kids with autism to temporarily show less intense symptoms.

Researchers hypothesize that fever may restore nerve cell communications in regions of the autistic brain. The restoration may help children improve socialization skills during a fever.

This is incredibly odd, but it could explain part of the reason so many mothers are convinced that vaccinations cause autism. It may be that the fever caused by vaccinations actually helps alleviate the symptoms of autism so that they are then far more obvious when it fades. Though, the study sounds very preliminary and, even if this effect proves to be constant, there have been so many reports that I'm not sure the effort could account for it.