At least 116 people, mostly children, have been confirmed with the polio-like condition called acute flaccid myelitis, federal health officials said Monday.

The Centers for Disease Control and Prevention says it is investigating 286 cases of AFM, which puts 2018 on track to be a busy year for cases of the little-understood syndrome.

“So far in 2018, there are 116 confirmed cases of AFM in 31 states,” the CDC said in its latest update.

The CDC says it still does not have a confirmed cause for AFM, which is marked by muscle weakness or paralysis caused by damage to the spinal cord. But the damage is the type often caused by viruses. Although the majority of patients have not tested positive for one specific virus, a family of viruses called enteroviruses are a prime suspect — especially one called EV-D68.

It is not linked to vaccines and it is not caused by polio, the CDC says.

Enteroviruses frequently circulate in fall and winter and while they rarely cause symptoms worse than a common cold, they can lead to complications in rare circumstances.

The CDC has noted an every-other-year pattern in cases of AFM, with 120 confirmed cases in 2014, 22 cases in 2015, 149 cases in 2016 and 33 cases in 2017.

“Respiratory illnesses and fever from viral infections such as enteroviruses are common, especially in children, and most people recover. We don’t know why a small number of patients develop AFM, while most others recover,” the CDC said.

Possible explanations include direct attack of the nerves by the virus, an immune response that affects nerves, or a genetic susceptibility among some patients.