Samples from two of the children tested positive for enterovirus 68, a rare virus that has been linked to severe respiratory illness in the past. Samples from the other three children were not collected or tested soon enough to yield conclusive results, said Dr. Emmanuelle Waubant, a neurologist at the University of California, San Francisco.
The poliovirus has been eradicated in the United States for more than 30 years. Only three countries in the world are not yet free of the disease: Afghanistan, Pakistan and Nigeria, according to the World Health Organization.
Why polio hasn't gone away yet
Enterovirus 68 was first identified in a California lab in 1962, after four children came down with a severe respiratory illness. Between 1970 and 2005, only 26 cases of enterovirus 68 in the United States were reported to the CDC. Since 2000, the government agency has kept a closer watch and has seen 47 cases, Oberste said. Outbreaks have occurred over the years in Asia and Europe, but it's still one of the rarest types of enterovirus.
More common -- and more concerning to health officials -- is enterovirus 71, which was discovered by the same California lab in 1969, Oberste said. Enterovirus 71 is usually associated with severe neurological issues, including aseptic meningitis, polio-like paralysis and encephalitis.
According to a CDC report, several outbreaks of paralysis caused by enterovirus 71 were seen in Europe in the 1960s and 1970s. In the late 1990s and early 2000s, fatal encephalitis was a big problem in Malaysia and Taiwan.