As AFM cases have surged in the fall — the third spike since 2014 — federal health officials and clinicians across the country have raced to understand the underlying cause of the disease. The CDC has established a new task force that is meeting in Atlanta this week to study the condition and come up with fresh leads on the factors behind it and how to treat it.

AFM is caused by inflammation of the spinal cord that results in severe muscle weakness and paralysis, often in the limbs. More than 400 confirmed cases, most of them in children, have been reported in the past four years, and one child with AFM died last year. Clinicians say the condition is underdiagnosed and underreported.

Competing theories have emerged about what triggers AFM. Several experts say there is a strong correlation between AFM and outbreaks of a common respiratory virus called enterovirus D68. Some enteroviruses are known to attack the spinal cord. The CDC is also looking into other possible causes, including whether AFM is caused not by a viral infection but by the patients' immune systems.

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Without clear guidance, doctors are trying several kinds of treatment. While some children have seen improvement, no treatment has been found to lead reliably to full recovery.

The CDC has seen a pattern of every-other-year spikes of AFM. In 2014, there were 120 confirmed cases; in 2016, there were 149 cases.

The CDC says states will continue reporting possible AFM cases, and medical experts will continue to review them and determine if they are cases of AFM. The number of confirmed cases by state will be updated.