The Centers for Disease Control and Prevention so far this year has confirmed 116 cases of acute flaccid myelitis, a rare but serious disease that can cause polio-like paralysis.
These 116 confirmed cases, out of a total of 286 possible cases reported to the CDC, have spanned 31 states. Last year, the CDC counted 33 confirmed cases in 16 states.
Acute flaccid myelitis affects gray matter in the spinal cord, causing muscles and reflexes to become weak and sometimes paralyzed.
The CDC said the cause of the illness is unknown.
More than 90 percent of people who experienced acute flaccid myelitis had a mild respiratory illness or fever consistent with a viral infection before they developed it, the CDC said. Viral infections are common among children, possibly helping to explain why more children contract the condition than adults.
Still, the agency doesn't know why a small number of people develop acute flaccid myelitis while most others recover. The CDC said it is continuing to investigate.