NEW YORK (AP) — Health officials are investigating nine cases of muscle weakness or paralysis in Colorado children and whether the culprit is a virus causing severe respiratory illness across the country.

The Centers for Disease Control and Prevention on Friday sent doctors an alert about the polio-like cases. The germ — enterovirus 68 — was detected in four of eight children tested.

Health officials don’t know whether the virus caused any of the children’s arm and leg weaknesses or whether it’s just a germ they coincidentally picked up.

The cases occurred within the last two months — during the same period the nation has seen a wave of severe respiratory illnesses linked to the virus.