Doctors say they are baffled by what’s causing an uptick in cases of a paralyzing, polio-like condition among kids across the U.S.

The illness, called acute flaccid myelitis, appears to be on the rise in 2018 after causing just a handful of cases in 2017. The Centers for Disease Control and Prevention says it’s investigating 127 reported cases, 62 of them confirmed to be AFM in 22 states.

“We have not been able to find the cause of the majority of AFM cases,” the CDC’s Dr. Nancy Messonnier told reporters on Thursday.

“Despite extensive laboratory testing, we have not determined what pathogen or immune response caused the arm or leg weakness and paralysis in most patients,” Messonnier added.

“We don’t know who may be at higher risk for developing AFM or the reasons why they may be at higher risk.”

Why is it so difficult to figure out?

There are not many cases

Since the first wave of AFM cases in 2014, the CDC has confirmed 386 cases of AFM. That is not a large group for study. “Overall, the rate of AFM over the years that it has been diagnosed which is since 2014 is less than one in a million. That is why we say that this disease is incredibly rare,” Messonnier said.

The syndrome is similar to polio, which once swept the country in regular late-summer epidemics, paralyzing thousands of children. But whatever is causing AFM, it does not seem to cause paralysis or muscle weakness to anywhere near the degree that polio did, said Dr. Benjamin Greenberg, an expert in rare neurological autoimmune disorders at the University of Texas Southwestern Medical Center, who has treated some AFM patients.

“Back at its peak for every 100 to 1,000 people we saw infected with polio, we saw somebody paralyzed,” Greenberg told NBC News. “We think with these viruses, it is a fraction of that.”

People are generally not tested for viruses

In 2014, the chief suspect for causing AFM was a virus called EV-D68. It was circulating at the same time that AFM cases started appearing and some of the affected children were infected with EV-D68. But others were not.

EV-D68 is an enterovirus, a distant relative of the poliovirus. But there are many enteroviruses, and they usually cause symptoms similar to the common cold, as do rhinoviruses, adenoviruses and many other viruses. There are hundreds of species of virus that infect people and many cause common cold-like symptoms.

Most people don’t even go to the doctor with a virus, and doctors rarely test patients to see which virus they have. So unless there are mass outbreaks of viruses that put people into the hospital with serious diseases, doctors and public health officials rarely know what viruses are “going around”.

Since 2014, the CDC says, there has not been any especially notable increase in cases of EV-D68, even as cases of AFM rise and fall. However, doctors do not routinely test people for it, either, so it is possible cases have been missed.

Because doctors do not know what virus may be responsible – if it really is a virus – there is no way to vaccinate people against it.

The CDC is open to the possibility that it's not a virus that is causing the condition, but cannot find any other plausible cause, either.

By the time they develop AFM, patients may have cleared the virus

It may take a few days after an infection for a patient to develop what is called neuroinvasive disease – when a virus attacks nerves, the spinal cord or the brain. A child may get a runny nose or a slight fever, and in most cases there would be no reason for parents to worry – until the tell-tale symptoms of a droopy eyelid, or weak arm, or trouble standing showed up. By then, the body may have cleared the infection itself and tests may not turn up any specific viral cause.

Most of the AFM cases that the CDC has studied have no indication of any particular infection. That doesn’t mean there was no infection – just that it has already been cleared by the immune system, or, possibly, that the illness is being caused by something doctors haven’t thought to test for.

Messonnier said other viruses besides EV-D68 have been found in patients with AFM. “But if you are having the peaks of disease every late summer and early fall you would think we (would be) finding a single agent. That is what we are not finding,” she said.

The CDC has been clear on one thing: none of the patients has tested positive for polio. In the past, an oral polio vaccine could cause a polio-like syndrome in some people, but that vaccine has not been available in the U.S. since 2000 and the last known case in the U.S. was in an infant with other health problems who was vaccinated in India, brought to the U.S. and died in 2013, the CDC says.

There are many different viruses, and they can cause many different neurological symptoms

“There are multiple different viruses that can cause this syndrome,” Greenberg said.

Just about any virus can cause neuroinvasive disease, but they rarely ever do. Zika is one of the most notorious recent examples. When it infects pregnant women, the Zika virus can go into the brains of the developing fetuses and cause permanent damage.

Just this past week the CDC reported on viruses spread by ticks and mosquitoes that can cause neurological disease. They include West Nile virus, Jamestown Canyon virus, Lacrosse virus, Powassan virus, St. Louis encephalitis and Eastern equine encephalitis. These viruses caused symptoms including encephalitis, a life-threatening inflammation of the brain; acute flaccid paralysis, which can include AFM and similar conditions; and meningitis.

This is why the CDC’s advice for preventing an infection that might lead to AFM is so general: wash your hands, and cover up and use repellent to prevent insect and tick bites.