A mysterious polio-like disease has sickened as many as 127 people in the U.S.

U.S. health officials are investigating an outbreak of a mysterious, polio-like disease that causes weakness in one or more limbs. The rare disease — acute flaccid myelitis, or AFM — has sickened 62 people, mostly children, in 22 states so far this year and is suspected in 65 more cases, the U.S. Centers for Disease Control and Prevention announced October 16.

Starting with an outbreak of 120 cases that brought the disease to national attention in 2014, close to 400 cases have been confirmed in the United States. So far, the CDC has been unable to figure out what’s causing the outbreaks. “This is a mystery,” Nancy Messonnier, director of the CDC’s National Center for Immunization and Respiratory Diseases in Atlanta, said during a news briefing. “We haven’t solved it yet.”

Although the disease is frightening, fewer than one in a million people in the United States get AFM every year, based on CDC data collected since 2014. “Parents need to know that acute flaccid myelitis is very rare, even with the increase in cases that we’re seeing now,” she said. Even so, the CDC recommends that patients who develop sudden weakness in their arms or legs seek immediate medical care.

What is acute flaccid myelitis?

The disease targets the spinal cord, leading to arm or leg weakness and loss of muscle tone. Symptoms can also include neck pain, headache, difficulty speaking, swallowing or breathing; the most serious symptom is respiratory failure. Children with AFM often start with a fever and respiratory illness before suddenly losing the ability to move one or more of their limbs. The paralysis can worsen quickly, which is why doctors recommend getting medical care at the first sign of unusual limb weakness.

What causes AFM?

The CDC is investigating viruses and environmental toxins as possible causes, but has yet to find a single agent responsible for the peaks in cases this year and in 2014 and 2016, when 149 cases were confirmed. Infections with viruses such as West Nile Virus, adenovirus, poliovirus and non-polio enteroviruses have led to symptoms that match those of AFM, the agency says. However, testing has shown that the U.S. cases since 2014 have not been caused by poliovirus.

Is there a treatment for AFM?

There is no cure or specific medical treatment for AFM. Doctors will try to relieve symptoms and may recommend physical or occupational therapy for muscle weakness.

Are there factors that increase the risk of AFM?

That still isn’t known. The disease tends to show up in the late summer and fall. Some patients who develop AFM recover; but many others have continued weakness and need ongoing medical care.