Two more children, one each in Pennsylvania and New Jersey, have been confirmed to have acute flaccid myelitis — a rare type of sudden muscle weakness that has been likened to polio.

That brings the total number of cases to seven in Pennsylvania and four in New Jersey this year, according to updates this week from the health department in each state.

Physicians say a primary cause of the condition is a type of virus called enterovirus D68, though the vast majority of people infected with the virus do not experience muscle symptoms. In those who do develop weakness or paralysis, determining the cause has been difficult because the virus can be hard to detect and has not been found in some patients.

On Monday, the U.S. Centers for Disease Control and Prevention said it had confirmed 80 cases of acute flaccid myelitis nationwide. The two new cases in Pennsylvania and New Jersey were identified after that. To be counted in the total, patients must suffer sudden muscle weakness or paralysis, typically in an arm or leg, along with evidence of damage to the gray matter of the spinal cord.

Typically, the symptoms arise after several days of a bad cold. That's what happened with Scarlett Camburn of Havertown, who suffered paralysis in her right arm three days before her second birthday in 2016.

"She literally went down for a nap and was fine, and she woke up and completely lost the use of her arm," said Scarlett's mother, Andrea Camburn, who has blogged about the experience.

Scarlett has recovered much of the function in that arm after surgery at Children's Hospital of Philadelphia, and she has another procedure in store next year.

Polio is caused by a different type of enterovirus and was far more widespread, striking thousands of patients each year before it was eradicated decades ago in the United States through vaccination.

Acute flaccid myelitis remains rare, with outbreaks occurring every two years: 120 cases in 2014, 149 cases in 2016, and at least 80 so far this year.