The Centers for Disease Control and Prevention has confirmed 62 cases of the rare polio-like neurological condition, also known as AFM, so far this year in the U.S. More than 90 percent of the cases involved children 18 or younger, with an average age of just 4 years old.
Cases have been confirmed in 22 states. Officials said they are looking at an additional 65 possible cases of AFM.
AFM is an illness that affects the nervous system, specifically the area of spinal cord called gray matter. It causes the muscles and reflexes in the body to become weak or even paralyzed. Cases of AFM are characterized by a sudden onset of arm or leg weakness and loss of muscle tone and reflexes.
Its symptoms are likened to those caused by polio, which was eradicated in the U.S. thanks to the polio vaccine. The CDC stressed that none of the children who developed these symptoms had the polio virus.
Additional symptoms can include facial drooping or weakness, difficulty swallowing and slurred speech.
Health experts say the disease can lead to paralysis and even death, but no deaths have been reported so far this year.
"We know this can be frightening for parents," Dr. Nancy Messonnier, director of CDC's National Center for Immunization and Respiratory Diseases, said in a press briefing on Tuesday. "I encourage parents to seek medical care right way if you or your child develops sudden weakness or loss of muscle tone in arms and legs."
AFM is extremely rare, with the CDC estimating that less than one in a million people in the United States will get AFM every year.
Still, officials saw an uptick in cases in 2014 and 2016, with peaks around late summer and fall. That pattern appears to be repeating this year.
Medical experts aren't sure what causes AFM, but it can occur as a result of a variety of viral illnesses including enteroviruses, West Nile virus, and adenoviruses.
"This is truly a mystery disease," Messonnier told CBS News. "We actually don't know what is causing this increase. For some of the previous cases we've identified one pathogen or another, but we have no unifying diagnosis."
Many times it can start with what looks like a respiratory illness, a little bit of a fever.
Messonnier said that of the cases confirmed this year, none have been related to the polio virus or West Nile virus. Several cases have been linked to enteroviruses or other germs, but officials have not been able to find a cause for the majority of these cases.
The CDC also does not yet know who may be at a higher risk for developing AFM or the long-term consequences of the condition. Some patients recover completely, while others continue to struggle with muscle weakness.
The CDC knows of one death in a child who had AFM in 2017, Messonnier said.
There is no specific treatment for AFM, but doctors may recommend physical or occupational therapy to help with arm or leg weakness.
Researchers at Children's Hospital Los Angeles areaimed at helping patients with AFM regain movement.
Messonnier said parents should be aware of the symptoms and seek medical care if they notice sudden muscle weakness in their children. They can also help prevent AFM by taking steps to protect children from serious disease, including frequent hand washing, staying up to date on immunizations, and preventing mosquito bites.