The number of children being stricken by a mysterious paralyzing condition continues to increase, federal officials say.At least 252 cases of acute flaccid myelitis, or AFM, have been reported to the Centers for Disease Control and Prevention so far this year from 27 states, including 90 that have been confirmed through Nov. 9, the CDC reported Tuesday.Most of the cases have occurred among children between the ages of 2 and 8.The illness usually starts as a fever and seemingly routine respiratory symptoms. But in some cases — between three and 10 days later — children suddenly suffer paralysis.The cause of the condition remains a mystery. But officials say there is a possibility it is being caused by a virus that affects the digestive system known as an enterovirus, though that remains just a theory.Another possibility is that the condition is being caused by an overreaction of the immune system to an infection. Also under investigation are rhinoviruses, which cause the common cold and other respiratory illnesses.Regardless, public health officials say they are racing to try to determine the cause and the best treatments.”As a mom, I know what its like to be scared for your child. And I know parents want answers,” Dr. Nancy Messonnier, director of the CDC’s National Center for Immunization and Respiratory Diseases, said during a briefing for reporters. “We’ve learned a lot about AFM since 2014, but there are things we still don’t understand.”No one has died from the condition so far this year, she noted, adding the condition remains rare.In about half of cases, the patients eventually fully recover. But the other half appear to be left with permanent disabilities, Messonnier says.AFM outbreaks appear to occur every two years, with most cases occurring in the late summer and fall. The pace of cases this year appears to be on track to have the same number as in the past few outbreaks, Messonnier says.While environmental toxins remain a possibility, Messonnier says that seems unlikely given how widespread the cases are.She also encouraged parents to continue to get their children vaccinated, saying vaccines don’t appear to be a possible cause. There is no evidence the condition is being caused by the polio virus.”We are trying to figure out what the triggers are that would cause AFM,” Messonnier says. Copyright 2018 NPR. To see more, visit http://www.npr.org/.