Had a co-worker ask me this today, and my initial response was that it was also known as Scarlet Fever. I had her going for awhile.
Rett Syndrome "is most often misdiagnosed as autism, cerebral palsy, or non-specific developmental delay," but "is a unique neurodevelopmental disorder which begins to show its affects in infancy or early childhood. It is seen almost exclusively in females, although it can occur rarely in boys. It is found in all racial and ethnic groups throughout the world." You can learn more at the International Rett Syndrome Foundation's website.