A puzzling condition that causes partial paralysis in children is back on the uptick in parts of the U.S., with six cases under investigation in Minnesota and 14 reported in Colorado, health officials said Monday.

The children have acute flaccid myelitis or AFM, a weakening of the nerves that resembles polio. It was first widely recognized in 2014, when 120 children were diagnosed.

So far this year, the Centers for Disease Control and Prevention says there have been 38 confirmed cases of the polio-like condition across 16 states.

There’s no cure and some children appear to have long-term disabilities, while others recover completely or nearly completely. A few need ventilators to breathe.

Acute flaccid myelitis appears to be caused most often by viruses, especially one called EV-D68. But other viruses may also be to blame. It seems to wax and wane from one year to the next, the CDC says.

After the 120 cases in 2014, there were just 22 in 2015. Then, cases spiked again to 149 in 2016. There were only 33 reported cases last year, according to the CDC.

That suggests 2018 could be another year in which more cases are reported, said Dr. Kevin Messacar, an infectious disease physician at Children’s Hospital Colorado.

“EV-D68 has been circulating in what looks like an every-other-year pattern,” Messacar told NBC News. “That makes it seem like in 2018 it could come back.”

Colorado has had 14 AFM cases so far this year, according to Colorado state epidemioloogist Dr. Rachel Herlihy. At least nine of them have been linked to a virus called EV-A71.

While aspects of the condition are puzzling, Messacar says it’s no mystery disease.

“It is important to know that even though this is a rare condition and is being reported as a mystery illness, we really do understand much more about the causes of this disease,” he said.