Cases of a paralyzing condition that affects mostly children appear to be on the rise across the country, an unofficial survey by NBC News has found.

The condition, called acute flaccid myelitis or AFM, appears to be caused by a viral infection, but health officials have been unable to link cases with any single specific virus. It causes symptoms ranging from muscle weakness to complete paralysis.

Health officials in 26 states tell NBC News they are investigating or have reported 87 cases of AFM.

It comes on suddenly and can cause a variety of symptoms, including dizziness, inability to walk, trouble swallowing or trouble moving an arm. There is no specific treatment but if children show symptoms, they need quick care — especially if there is trouble breathing, as they may need a ventilator.

As of the end of September, the Centers for Disease Control and Prevention had confirmed 38 cases in 16 states. CDC did not name the states, but health officials in Colorado said they had 14 cases in 2018 and officials in Minnesota confirmed six.

State health officials in 24 other states have since told NBC News they have both suspected and confirmed cases as well, some of them reported after the end of September. An unofficial tally based on interviews with these officials shows 35 suspected and 52 confirmed cases of acute flaccid myelitis, for a total of 87 suspected and confirmed cases.

“There are currently two possible cases reported in 2018 that are under investigation,” Brittany Fowler, a spokesperson for the Maryland health department, told NBC News.

“We just had a suspected case reported in an adult male,” said Lynn Sutfin of the Michigan Department of Public Health.

A spokeperson for the CDC said the agency did not plan to release updated numbers this week.

It takes time to confirm a case of AFM. Viruses can cause a range of neurological symptoms and AFM is one of the rare conditions that can follow a viral infection. For AFM to be confirmed, doctors must determine whether the spinal cord is affected.

The CDC has a list of tests to be run to confirm a case.

“All suspected cases are sent to the CDC for review by their neurologists,” said Lara Anton of the Texas Department of State Health Services. Texas has confirmed eight cases of AFM this year.

“It takes about a month for them to review a case and make the determination of AFM,” Anton added.

“There are a few cases undergoing that process right now that are not reflected in the official case count. All but one of the Texas cases are children.”

Alabama is investigating two possible cases, also. “We haven’t completed our own investigations yet to confirm it’s AFM so we haven’t reported to the CDC,” said Sherri Davidson, Alabama’s state epidemiologist. “It may take a month or two because there’s a lot of documentation to go through and an expert panel that needs to review.”

Health officials will never have a firm count of cases of AFM because not all states require doctors to report them.

South Carolina, for instance, does not require physicians to report cases, although clusters of cases — several occurring at the same time in the same area — would be reported, said Tommy Crosby of the state Department of Health and Environmental Control or DHEC.

“Providers will often consult with DHEC when they suspect AFM as a complication of an infectious disease,” Crosby said.

AFM looks very much like polio.

“The patients’ symptoms have been most similar to complications of infection with certain viruses, including poliovirus, non-polio enteroviruses, adenoviruses, and West Nile virus,” the CDC says.

Other viruses have also been linked with cases, including one called EV-A71. Both EV-D68 and EV-A71 are enteroviruses and are distant relatives of polio.

New York reports an outbreak of EV-D68, with 39 reported cases, and has two suspected cases of AFM.

"EV-D68 infection most commonly results in mild symptoms such as runny nose, sneezing, cough, body aches and muscle aches," the state health department said in a statement.

"Severe symptoms, while less common, may include wheezing and difficulty breathing. In some rare instances, the virus can cause acute flaccid myelitis (AFM), a serious condition that causes weakness in the arms or legs. However, there are other causes of AFM besides EV-D68 and severe respiratory illness is a greater concern with this virus."

The virus that causes polio has been eradicated in most of the world by vaccination, but regular epidemics once swept through, paralyzing thousands of children. Afghanistan and Pakistan are the only two countries where wild poliovirus has been seen this year, because conflict makes it difficult to vaccinate all children.

In the U.S., “the increase in AFM cases in 2014 coincided with a national outbreak of severe respiratory illness among people caused by enterovirus D68 (EV-D68). Among the people confirmed with AFM, CDC did not consistently detect EV-D68 in every patient,” the CDC said.

There are no vaccines against either EV-D68 or EV-A71.

Since 2014, cases have risen and fallen. The CDC reported 120 cases of AFM in 2014, 22 cases in 2015, 149 cases in 2016 and 33 cases in 2017.

“Surveillance has shown us that AFM cases generally peak in the months of September and October,” the Nevada state health department said in a bulletin sent to healthcare providers in August.

“A biennial pattern has been observed, with the majority of cases reported in 2014 and 2016, and smaller numbers reported in 2015 and 2017. If this pattern continues, we should expect to see an increase in AFM cases in 2018.”

The CDC says to wash hands regularly and thoroughly to protect against bacterial and viral infections, in general.

This story has been updated the reflect the latest case count.