The Centers for Disease Control and Prevention warned parents and caregivers Tuesday to watch for an uncommon, polio-like condition that mostly strikes children, usually between August and November.

Acute flaccid myelitis, which may be caused by any of several viruses, is marked by sudden weakness or paralysis in the limbs. Since surveillance began in 2014, prevalence of the syndrome has spiked every two years in even-numbered years, often afflicting children about 5 years old.

The disease is very rare, but a quick response is critical once the weakness sets in; the disease can progress over hours or days and lead to permanent paralysis or respiratory failure, according to a report issued Tuesday by the CDC. Among 211 patients who were struck by the disease in 2018, 98 percent were hospitalized, 54 percent required intensive care and 23 percent were placed on ventilators.

Limb weakness, difficulty walking and limb pain are often preceded by fever or respiratory illness, usually by about six days, the CDC said. Hundreds of U.S. children have been affected and many do not fully recover.

A number of viruses – including West Nile virus, adenovirus,and nonpolio enteroviruses – are known to produce the symptoms in a small number of people, who become infected by those pathogens. But enterovirus, particularly one dubbed EV-D68, appears to be the most common cause, the CDC said. The National Institute of Allergy and Infectious Diseases is working on a vaccine for EV-D68.

Thomas Clark, deputy director of CDC’s division of viral diseases, said the coronavirus pandemic may force doctors to evaluate patients by phone or telemedicine but warned they should not delay if they suspect the syndrome, which is considered a medical emergency.

It’s unclear whether mask wearing, social distancing and other measures that have been taken against the coronavirus will limit the outbreak of acute flaccid myelitis expected this year.