Cases of a rare illness called Acute Flaccid Myelitis (AFM) that has affected more than a hundred Americans this year appears to have peaked, the Center for Disease Control and Prevention (CDC) said Monday.

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“Based on the number of reported cases of AFM in the United States through November 30, 2018, it appears that the number of persons under investigation (PUIs) for AFM has peaked and will continue to decline for the remainder of 2018,” federal health officials said in a statement.

Of the 299 suspected cases of AFM reported to the CDC in 2018, the health agency has confirmed 134 cases of the illness in 33 states. The latest confirmed cases were in September and October.

The CDC said it has seen an increase of AFM cases every other year since 2014. Mostly young children have been affected.

AFM is a "polio-like" illness that is rare but serious. It primarily impacts the central nervous system, “specifically the area of the spinal cord called gray matter, which causes the muscles and reflexes in the body to become weak,” the health agency says online.

The illness can be caused by certain enteroviruses, or “a group of viruses that cause a number of infectious illnesses which are usually mild,” the European Centre for Disease Prevention and Control says online. For the past four years, the CDC said 90 percent of AFM patients “had a mild respiratory illness or fever consistent with a viral infection before they developed AFM.”

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The poliovirus and the West Nile virus have also been known to cause AFM. Treatment for the condition varies.

“Although fewer cases are expected in coming months, CDC and partners continue to carefully study AFM to gain new understanding of the condition so that we can better diagnose, treat, and prevent it in the future,” the CDC said.