Scientists have found the strongest evidence yet that a virus is to blame for a mysterious illness that can quickly paralyze children.

The very rare condition, called acute flaccid myelitis or AFM, is similar to the polio disease. The first reports of the disease came from the American state of California in 2012. Since then, the U.S. has experienced an increasingly bigger outbreak every other year, from late summer into autumn.

Doctors have long believed that certain viruses known as enteroviruses caused AFM. But they did not have enough evidence to prove it.

So researchers tried something new. They checked patients' spinal fluid for signs the immune system had fought an invading virus. Children who got sick had antibodies that target enteroviruses.

Dr. Michael Wilson is with the University of California, San Francisco. He helped lead the research. He said the evidence did not prove their idea. But it was a powerful sign enteroviruses cause AFM.

His team reported the findings in the publication Nature Medicine.

Study co-writer Dr. Riley Bove is with the same university as Wilson. Bove's son developed AFM at age four. He noted, "If you don't have a cause, you can't have a vaccine." Bove added that Wilson developed "a good enough microscope, in a sense, to find things they suspected were there."

About 590 cases of the illness have been confirmed in the U.S. since 2014. Cases increased that year, in 2016 and in 2018. Only a few were reported in the years in between. So far, there have been 22 cases this year.

Bove's son Luca demonstrates the pattern. His whole family caught a cold in the summer of 2014. A few days later, Luca woke up with weakness in his neck that traveled down his shoulder. Within days, he had body-wide paralysis and trouble breathing. He recovered slowly. Today, he still has some paralysis in his neck, shoulder and arm.

Experts believe either a germ or the body's reaction to a germ damages nerves in the spinal cords of patients like Luca. They are preparing for another possible increase of cases next summer.

I'm Jonathan Evans.

Lauran Neergaard reported this story for the Associated Press. Jonathan Evans adapted it for Learning English. Kelly Jean Kelly was the editor.

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antibodies – n. substances produced by the body to fight disease

germ – n. a very small living thing that causes disease

immune – adj. not capable of being affected by a disease

outbreak – n. a sudden start or increase of fighting or disease

paralyze – v. to make a person or animal unable to move or feel all or part of the body

spinal – adj. of, relating to, or affecting the spine