ATLANTA, Georgia (CNN) — The U.S. Centers for Disease Control and Prevention reported Monday that there are 155 patients under investigation this year for acute flaccid myelitis, a condition that that can cause paralysis and mostly affects children.

Of these, 62 have been confirmed by the CDC in 22 states, and the remainder continue to be investigated.

Acute flaccid myelitis, also called AFM, is a rare but serious condition that affects the nervous system — specifically, the area of the spinal cord called gray matter. It affects fewer than one in a million people each year across the country, the CDC estimates.

The number of patients under investigation is up from 127 patients a week ago, though no new confirmed cases have been reported.

The average age of patients confirmed to have the condition is just 4 years old, and more than 90% of cases overall occur in children 18 and younger, according to Dr. Nancy Messonnier, director of the agency's National Center for Immunization and Respiratory Diseases.

"CDC has been actively investigating AFM, testing specimens and monitoring disease since 2014, when we first saw an increase in cases," Messonnier told reporters last week. "Most AFM cases occur in the late summer and fall," but no geographic clustering has been found and there is no other "unifying factor to explain the peaks," which seem to occur every other year, she added.

The CDC received information on 33 confirmed cases of AFM in 2017, 149 cases in 2016, 22 cases in 2015, and 120 cases in the latter part of 2014.

"There is a lot we don't know about AFM," including the cause in a majority of cases, Messonnier said. While potential causes may include certain viruses, environmental toxins and genetic disorders, the CDC says, "AFM can be difficult to diagnose because it shares many of the same symptoms as other neurologic diseases."

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It is also unclear who could be at higher risk of developing AFM, Messonnier said. The CDC does not fully understand long-term consequences or why some patients recover quickly while others continue to experience paralysis and weakness.

The CDC urges parents to be aware of this illness and to seek medical care right away if family members develop sudden weakness or loss of muscle tone in the arms or legs.

"This is a mystery so far, and we haven't solved it yet, so we have to be thinking broadly," Messonnier said.

'Not knowing'

"What makes it worse is not knowing what caused it," said Erica Palacios, whose 2-year-old daughter, Abigail, began showing signs of AFM last month.

"It was one of the most horrific experiences of my life," said Palacios, of Columbus, Georgia.

All four of her children had come down with a cold at the same time, but young Abigail's condition continued to worsen despite her mother's best efforts to keep her kids healthy.

After a few days, she noticed that Abigail's arm had gone limp and rushed her daughter to the hospital, where doctors initially drew blanks, she said, adding that at one point Abigail couldn't move below the neck.

"They ran every test known to mankind," Palacios said.

Doctors transferred Abigail to Children's Healthcare of Atlanta, where she required breathing and feeding tubes in the ICU. She was later transferred to the healthcare system's Inpatient Rehabilitation Program at Scottish Rite hospital, where she regained some movement in one arm, and can now hold her head up and walk with assistance.

"At this point, she is making almost daily gains," said one of Abigail's doctors, pediatric rehabilitation physician Dr. Laura Jones.

Still, the 2-year-old will be returning home in a wheelchair. Palacios said her daughter wants to get up and play, but gets frustrated and doesn't understand why she can't.

"We don't know a lot about the long-term prognosis of AFM right now. That's something that we're still really learning about," Jones said. "We know that some kids have great recovery and recover really quickly, and other kids continue to have a lot of weakness going forward, but we haven't determined what factors decide which kids fall into which category yet."

Palacios says she wants answers to what causes AFM, and how to prevent and treat it — for Abigail, and for other families who have been affected by it.

"She was at that age where was just learning how to be independent, and now she's dependent again," Palacios said.

"No parent should ever have to experience that."

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