The first known case of this polio-like illness - 2012, Sofia Jarvis of Berkeley. Neurologists found four other patients who developed paralysis between August 2012 and July 2013 - but no one seems to know what's causing it.
Here's what doctors do know:
-children show signs of paralysis and breathing problems
-all reported cases are in California
-polio is a highly infectious disease that invades the nervous system and, in one in 200 cases, causes irreversible paralysis.
-all of the kids had a polio vaccine -- and testing this *isn't* not polio.
Doctors say the culprit is an enterovirus - a family of viruses including polio but also some milder diseases. There isn't a vaccine for other enteroviruses.
"It's a terrible feeling . You would do anything to protect your child. So when there's something you cannot protect your child from, it's a horrible feeling," mother Karen Klein said.
For more than 30 years, there have been no cases of polio in the United States. According to the World Health Organization, only three countries in the world are not yet free of the disease: Afghanistan, Pakistan and Nigeria.
Doctors don't want parents to panic - calling this very rare.
"You don't have control over something like that and you want to do anything to protect your child. When you can't you feel helpless," Klein said.