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.