Editor’s note: This story was updated on Sept. 30 with additional cases in Colorado and Virginia and comments from the Children's Hospital Colorado media briefing.

Sept. 29, 2014 -- Some children hospitalized with breathing problems caused by enterovirus D68 in Colorado, Virginia, and Kansas City have also developed unexplained paralysis in their arms and legs, officials say.

Doctors have been watching for paralysis in enterovirus patients affected by the current outbreak, which the CDC says has spread to at least 40 states.

That’s because D68 is related to the virus that causes polio, which crippled thousands of children across the United States before a vaccine was created in the 1950s. And in 2012, researchers at Stanford University reported 25 childhood cases of paralysis of an unknown cause that shared features with polio, and in some cases also started as a respiratory illness.

When enteroviruses move beyond the gut or airways, they can attack nerve cells in the spine that control movement.

“The polio virus is a professional at doing this. All the other enteroviruses are amateurs. They do it very, very, very rarely,” says William Schaffner, MD. He's an infectious disease expert at Vanderbilt University in Nashville, TN. “But D68 is a new one. It’s a bit of a different enterovirus. The question is: Could it be doing this?”

He cautions that other germs, including West Nile virus, can also cause paralysis, and it could simply be a case of double infection in this cluster of children.

“This could be just coincidental, so we can’t leap to the conclusion that enterovirus D68 is the cause of this paralysis,” he says. “It’s right at the top of our list of suspects, but we haven’t nailed it yet.”