Cases of a mysterious illness that causes children to become partially paralyzed have skyrocketed this year, with the highest number seen since 2012, health officials have warned. The Centers for Disease Control and Prevention has confirmed 158 cases of acute flaccid myelitis in 36 states this year. The cause of the condition—which affects the spinal cord’s gray matter and has left children unable to move their arms, legs, face, neck, and back—is unknown. According to the Associated Press, symptoms of the condition appear about a week after the child experiences a fever or respiratory illness. CDC officials have said many children with AFM will have “lasting paralysis,” and over half of the children diagnosed this year were taken to “hospital intensive-care units and hooked up to machines to help them breathe,” the AP reports. Over 300 suspected cases of the condition were reported in the U.S. this year, but no one has reportedly died from AFM in 2018. In 2017, there was one confirmed death from the condition and other deaths may have occurred in the past.