Acute flaccid myelitis: a rare condition of the nervous system that causes the muscles and reflexes of the body to weaken. Viruses, toxins, and genetic disorders can all cause acute flaccid myelitis. Viruses that can cause the condition or similar symptoms include poliovirus, non-polio enteroviruses, adenovirus, or West Nile virus. The exact cause of most cases is not definitively known. However, development of acute flaccid myelitis is extremely rare. It is estimated that less than one in 1 million people in the US develop this condition every year. Symptoms are a sudden onset of arm or leg weakness and loss of muscle tone and reflexes. Other symptoms can include facial droop, eyelid droop, difficulty moving the eyes, facial weakness, slurred speech, or problems with swallowing. Respiratory failure is the most serious potential symptom. Even though the condition is very rare, the US CDC has received reports of an increasing number of cases of acute flaccid myelitis since 2014; most of the reported cases have occurred in children.