Doctors in California are baffled by a mysterious illness that mimics the dreaded Polio virus. Polio paralyzed thousands of children and adults worldwide until it was eradicated in 1979.

The new polio-like illness has infected up to 25 children in California so far, CBS News reports. The new illness causes flu-like symptoms and muscle wasting, severe weakness or paralysis in the arms and legs, a new case study reports.

Cases of polio-like symptoms caused by an enterovirus have been reported in children in Asia and Australia over the past decade.

“We think one reason why these cases may have occurred in California is that we are at the western-most part of the United States, so we may have had a higher circulation of the virus that was originally identified in Asia,” said Dr. Emmanuelle Waubant, co-author of the case report and a neurologist at the University of California, San Francisco Medical Center.

Like the poliovirus, most people infected with the new strain have no symptoms.

“It’s very likely that the vast majority of people infected with enterovirus will be fine,” Waubant said. Most never develop neurological problems because the virus usually stays in the respiratory tract, where it may cause flu-like symptoms.

After discovering 5 new cases, researchers and public health officials in California checked previous cases to determine if there was a link to the new virus strain.

“These five new cases highlight the possibility of an emerging infectious polio-like syndrome in California,” Dr. Keith Van Haren, an instructor in neurology and neurological sciences at the Stanford School of Medicine in Palo Alto, Calif., said in a written statement Sunday.

Researchers point out that all five confirmed cases received polio vaccines in the past.

“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,” said Van Haren.

*(bold emphasis mine)