More children have been diagnosed with acute flaccid myelitis, the polio-like paralyzing illness, according to numbers released Monday by the US Centers for Disease Control and Prevention.
There have now been 106 confirmed cases of AFM in 29 states this year, according to the CDC, an increase of 16 since last week.
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There are also 167 possible cases of the illness, an increase of five from the previous week.
Since 2014, there have been 430 confirmed cases of the rare disease, and 90% have been children, according to the CDC.
AFM is a rare illness that affects the nervous system, especially the gray matter in the spinal cord, and causes muscle weakness and sudden onset of paralysis. There's a spectrum of how children can be affected: Some regain the use of their paralyzed limbs, while others are paralyzed from the neck down and can breathe only with the help of a ventilator.
There is no cure and no vaccine.
There is also no known cause. Although several neurologists who serve as advisers to the CDC say they feel sure that an enterovirus -- the same family of viruses that cause polio -- is most likely to blame, the CDC says it's still casting a net wide.
The CDC's Dr. Nancy Messonnier said last week that the agency is considering the possibility that an infectious pathogen is causing AFM but added that "we're broadening our hypotheses."
When asked whether a toxin or vaccines could be triggering AFM, Messonnier, director of the National Center for Immuniziation and Respiratory Diseases, replied that "we're not ruling anything out at this point."
About three to 10 days before becoming paralyzed, nearly all children who developed AFM experienced a viral illness with symptoms such as fever and cough, the CDC reported last week.
Viral illnesses are very common among children, and so it's not clear why only a relatively small number develop AFM. Even within the same family, several siblings can develop the same cold-like symptoms but only one becomes paralyzed.
In a CNN story last month, several of the CDC's medical consultants and parents of sick children criticized the agency for being too slow to respond to the outbreak.
Messonnier said last week that the agency had funded state health departments to increase physician awareness in identifying cases, increased its network of neurologists to assist with and confirm cases, and established an AFM task force of national experts.
CDC spokeswoman Kristen Nordlund also said last week that the agency had assigned 14 officers from the Epidemic Intelligence Service -- known as "disease detectives" -- to help review reports of AFM cases.
"As a mom myself, I can certainly understand why parents are worried. I am concerned about this increase in AFM," Messonnier said.