In 2018, there were 201 confirmed cases of acute flaccid myelitis, the polio-like condition affecting U.S. children, according to the CDC.
It is the highest number the CDC has reported since AFM cases first spiked in 2014.
Health departments are still investigating potential cases of AFM, and experts are still unsure of what causes it.
It also is unclear why the number of cases has been higher in alternate years, with 35 cases in 2017, 149 in 2016, 22 in 2015 and 120 from August to December 2014.
The CDC created a task force in 2018 to focus on bolstering the investigation into the current spate of AFM cases.
"We work closely with national experts, healthcare providers and state and local health departments to thoroughly investigate AFM by looking for possible risk factors and causes, figuring out why some people develop this condition, monitoring AFM activity nationwide and updating possible treatment options," the CDC said.