A rare, mysterious polio-like illness seems to be on the rise again in the U.S., with 62 cases confirmed so far this year to the federal government, as of Tuesday — including at least half a dozen in Pennsylvania and New Jersey.

Called acute flaccid myelitis, the disease is marked by weakness in one or more limbs, along with slurred speech, difficulty swallowing, and drooping of facial muscles in some patients. The condition is brought on by inflammation in the spinal cord, in some cases caused by a virus. But in many cases the cause is unknown, the U.S. Centers for Disease Control and Prevention says.

First widely acknowledged in 2014, the disease seems to ebb and flow on a two-year cycle, with an uptick in 2016 and again this year, according to an advisory from the Philadelphia Department of Public Health.

Most patients recover from the condition, though typically they need physical therapy. Severe cases can result in death.

Two patients with the condition were treated in August at Children's Hospital of Philadelphia, the hospital said Monday. It did not disclose where those patients live, but the city health department said no cases have been confirmed in Philadelphia residents. In previous years, physicians at CHOP have treated patients with the condition successfully with intravenous immunoglobulin to reduce swelling in the spinal cord, the hospital said.

Three cases have been reported this year in New Jersey — one in February and two in August, the state Department of Health said.

And last week, Children's Hospital of Pittsburgh confirmed it was treating three children with the condition, CBS station KDKA reported.

The 62 cases so far this year were reported in 22 states. Since 2014, 386 cases have been reported to the CDC.