Acute flaccid myelitis (AFM) is a rare illness that anyone can get. It affects a person’s nervous system, specifically the spinal cord. AFM can result from a variety of causes, including viral infections.

In a Centers for Disease Control and Prevention (CDC) update, from January 1 to October 31, 2016, a total of 108 people in 36 states across the country were confirmed to have AFM. This is up from 89 confirmed cases at the end of September.

CDC notes even with an increase in cases in 2016, AFM remains a very rare disease (less than one in a million).

CDC reports the following “knowns” about AFM since August 2014: Most patients are children; the patients’ symptoms have been most similar to those caused by certain viruses, including poliovirus, non-polio enteroviruses, adenoviruses, and West Nile virus and despite testing a variety of specimens, they have not consistently detected a pathogen in the patients’ spinal fluid.

CDC does not yet know the cause of the AFM cases.