The illness causes severe weakness or rapid paralysis in one or more limbs.
The Los Angeles Times reports that state public health officials have been investigating the illness since a doctor requested polio testing for a child with severe paralysis in 2012. Since then, similar cases have sporadically been reported throughout the state.
Dr. Carol Glaser, leader of a California Department of Public Health team investigating the illnesses, called the doctor's request "concerning" because polio has been eradicated in the U.S. and the child had not traveled overseas.
The symptoms sometimes occur after a mild respiratory illness. Glaser said a virus that is usually associated with respiratory illness but which has also been linked to polio-like illnesses was detected in two of the patients.
Dr. Keith Van Haren, a pediatric neurologist at Stanford University's Lucile Packard Children's Hospital who has worked with Glaser's team, will present the cases of five of the children at the American Academy of Neurology's upcoming annual meeting.
He said all five patients had paralysis in one or more arms or legs that reached its full severity within two days. None had recovered limb function after six months.
"We know definitively that it isn't polio," Van Haren added, noting that all had been vaccinated against thatdisease.
Glaser wouldn't provide the number of illnesses. Van Haren said he was aware of around 20.
She urged doctors to report new cases of acute paralysis so that investigators can try to figure out a possible cause.