Acute flaccid myelitis (AFM) is a rare and serious neurological condition that causes certain muscles and reflexes to become weak (flaccid). The symptoms come on suddenly and can also affect your ability to breathe. AFM causes inflammation in the gray matter of your spinal cord. You have grey matter in your brain and spinal cord (central nervous system). Of all the types of central nervous system tissue, it plays the most significant part in allowing you to function normally. Gray matter in your spinal cord specifically plays a role in controlling movement. AFM is a relatively new diagnosis — researchers first described it in 2014. Previously, healthcare providers and researchers thought it was a type of transverse myelitis.