A total of 90 people in 27 states have come down with the rare, polio-like neurological condition acute flaccid myelitis, also known as AFM, so far this year, the Centers for Disease Control and Prevention reported Tuesday. That's up from 72 cases reported last week. The CDC is investigating an additional 162 cases for potential AFM.

Almost all of the patients are children under the age of 18. Officials still do not know what causes the illness and what is behind the recent spike in cases.

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. In extreme cases, patients may need a ventilator to breathe.

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Its symptoms are likened to those caused by polio, which was eradicated in the U.S. thanks to the polio vaccine. However, the CDC has stressed that none of the children who developed these symptoms had the polio virus.

Health officials saw the first wave of AFM cases in 2014, when 120 cases were confirmed in the United States. Another 149 were reported in 2016. Case counts were far lower in 2015 and 2017, and experts don't know why it seems to follow an every-other-year pattern. In years where more cases have been reported, peaks are seen in the late summer and fall.

The CDC knows of one death in a child who had AFM in 2017. No deaths from the illness have been reported in 2018.

Questions also remain over what causes the illness itself. In 2014, the first wave of the mystery disease coincided with an outbreak of a specific type of virus, an enterovirus called EV-D68, leading the CDC to study a possible connection between the two.

However, this year officials have tested for EV-D68 in about three-quarters of the confirmed AFM cases and detected it in the spinal fluid of just one patient. Another type of enterovirus called EV-A71 was found in another patient.

Dr. Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases at the CDC, told reporters during a press briefing Tuesday that the CDC has created a task force of specialists to better understand AFM. She said the CDC is "broadening our hypotheses" for what may cause AFM.

For example, if the illness is caused by a virus, Messonnier said, it is possible that the currently available tests are unable to pick it up, or perhaps the germ has already cleared the spinal fluid by the time the testing was conducted, or that it is hidden elsewhere in the body.

Another theory is that there's something in certain individuals that predisposes their immune system to react so severely that it triggers paralysis.

All things considered, 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.

"As a mom myself I can certainly understand why parents are worried," Messonnier said. "But it's important for parents to realize it still is a relatively rare condition."

She says parents who are concerned about AFM or think their child has symptoms of the illness should speak to their pediatrician.