The CDC is monitoring the outbreak of an illness that causes severe weakness and limb paralysis, similar to polio.

An estimated 25 California children have developed a syndrome with polio-like symptoms, with five confirmed cases—three in the San Francisco Bay Area.

A new report from the American Academy of Neurology shows that a cluster of children developed the mysterious illness, which can cause paralysis of one or more limbs.

Case report author Dr. Keith Van Haren, an instructor of Neurology at Stanford University School of Medicine, said there are numerous potential viruses that can affect the spine, but few have been reported in the U.S. in recent history.

“In the past decade, newly identified strains of enterovirus have been linked to polio-like outbreaks among children in Asia and Australia,” he said in a statement. “These five new cases highlight the possibility of an emerging infectious polio-like syndrome in California.”

Polio was a major concern for generations of Americans, showing up in highly contagious outbreaks and causing paralysis, muscle deterioration, and sometimes death. Children began to receive the polio vaccine in 1954, and thanks to high rates of vaccination, the polio virus has essentially been eradicated over most of the planet.

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Three of the first five known cases began with a respiratory illness. Other symptoms peaked two days after onset.

Symptoms did not improve during treatment, and the children continued to experience poor limb function six months after contracting the disease.

All of the children affected were vaccinated against polio, and two tested positive for the enterovirus-68, a rare virus known to cause polio-like symptoms. No cause was given for the other three children, the researchers said.

Read: How Vaccines Prevented a Measles Outbreak in the Bay Area »

“Our findings have important implications for disease surveillance, testing, and treatment,” Van Haren said. “We would like to stress that this syndrome appears to be very, very rare. Any time a parent sees symptoms of paralysis in a child, the child should be seen by a doctor right away.”

Van Haren and his colleagues discovered the pattern after seeing similar cases at their medical centers. They reviewed data from all polio-like cases among children whose cases were reported to the California Department of Public Health’s Neurologic and Surveillance Testing (NST) program from August 2012 to July 2013.

Researchers looked for children who had experienced paralysis that affected one or more limbs, who had been monitored with abnormal magnetic resonance imaging (MRI) scans of their spine. Children affected with Guillain-Barre syndrome or botulism, which can have similar symptoms, were excluded from the study.

The California Department of Public Health is working with the U.S. Centers for Disease Control and Prevention (CDC) to collect data and monitor the progression of the disease.