“Federal health officials are urging doctors to quickly report any cases of a mysterious, polio-like illness that primarily hits children in an effort to understand the sickness before the next outbreak.

The illness, whose cause has perplexed government scientists, affected 233 patients in 41 states last year, and circulates primarily between August and October. It is known as acute flaccid myelitis, or AFM, and leads to limb weakness that can become so severe that children need to be hooked up to a breathing machine.