Recent reports of a mysterious illness with polio-like symptoms in a few California children is worrying and baffling medical experts. Polio had been declared eradicated in the United States over three decades ago.

Neurologists reported evidence of polio-like symptoms in five California children, but are still trying to determine the actual cause of the ailment. These kids developed paralysis in one or more of their limbs in the period between August 2012 and July 2013. All of them had been vaccinated against the poliovirus and treatment did not seem to have been effective in helping them regain motor function.

The research on the children was carried out by a team led by Stanford University pediatric neurologist Keith Van Haren and University of San Francisco neurologist Emmanuelle Waubant. In addition to the five who showed polio-like syndrome, another 15 to 20 children also showed similar symptoms but were not closely evaluated, though initial blood tests and other information provided evidence that they had also suffered sudden paralysis or extreme limb weakness and possible spinal chord damage. All the children were aged between 2 and 16.

Poliovirus is a member of the Picornaviridae family, which also includes enteroviruses and rhinoviruses (which causes the common cold). According to the Centers for Disease Control and Prevention, there are more than 100 types of enterovirus resulting in 10 million to 15 million infections in the U.S. each year. Two of the five affected California children have tested positive for enterovirus 68, which is a rare type of enterovirus. Paralysis is usually linked to enterovirus 71. Both these types of enteroviruses are rare in the U.S. and state federal officials believe the risk is low. Though about 270 cases of enterovirus 71 were reported in the country between 1983 and 2005, none caused a larger outbreak.

The specialists have cautioned health care providers and asked them to send in samples from patients showing similar symptoms. Laboratory tests check for poliovirus using throat secretions, stool samples, or cerebrospinal fluid.

Dr. Carol Glaser, chief of the Encephalitis and Special Investigation Section at the California Department of Public Health says that vaccination is the key to preventing polio from returning to the United States.