Researchers don’t know the exact cause of acute flaccid myelitis, but they think that viruses, especially non-polio enteroviruses, play a role in causing the condition. Most people with AFM had a mild respiratory illness (such as the common cold) or fever before they developed AFM. Researchers and healthcare providers have suspected enterovirus D68 and enterovirus A71 in many AFM cases. Enterovirus D68 most often causes a respiratory illness and circulates in the United States during the summer and fall every other year.