Acute flaccid myelitis has cropped up again after a 40-year absence in the United States. It might be part of an annual infectious cycle.

Scientists are perplexed about what has caused the reemergence of a rare virus that has caused a paralysislike disease in about four dozen children in the United States this year.

However, a medical expert interviewed by Healthline said the virus is reaching the end of its peak season and will probably fade away during the winter and spring months.

It may or may not be back next year.

“The risk will decrease and not return until next summer. There are probably a lot of other things to worry about,” said Dr. James Cherry, a distinguished research professor at the David Geffen School of Medicine at the University of California, Los Angeles (UCLA).

Doctors from the Centers for Disease Control and Prevention (CDC) have been monitoring an increase in rates of acute flaccid myelitis (AFM).

Although the illness can affect anyone, 90 percent of the recent confirmed cases have occurred in children.

CDC officials say they began receiving reports of AFM in August 2014.

“Since then, CDC has been actively investigating this illness. We continue to receive reports of sporadic cases of AFM,” CDC officials said in a statement.

In all, 120 cases of AFM were reported in the second half of 2014. In 2015, the number of cases dropped significantly, only to rise again in 2016.

This year, from January through Aug. 31, 50 people in 24 states across the country were confirmed to have AFM. So far, no cause for the increase in AFM cases has been found.

CDC officials note the disease is still rare, affecting less than 1 in 1 million people in the United States.

Read more: Treating children who have Crohn’s disease »

Cherry said AFM was first diagnosed in 1962.

It then disappeared for about 40 years before reemerging in Europe and Asia.

Two years ago, it cropped up again in the United States, first striking the Midwest.

AFM has been linked to viral infections, environmental toxins, genetic disorders, or Guillain-Barré syndrome.

Cherry said he and other medical experts believe AFM is caused by enterovirus D68.

He said the respiratory-based virus can be found primarily in a patient’s stool, but it can also be detected in the nose as well as on the skin.

It can be transferred by contact from person to person.

He said the recent surge in children’s cases may be because younger people don’t have antibodies against the virus.

However, he added, it’s more likely that adult cases haven’t been diagnosed while pediatricians have been diligent in diagnosing children.

He said there may have been a lot more children infected with the virus and only a small percentage came down with the paralysis symptoms.

Cherry said there are more than 100 enteroviruses. Every year, four or five of them seem to dominate during the summer and fall months.

“Why that happens, I don’t think anybody knows,” he said.

This may simply be a case of enterovirus D68 taking its turn at the top of the infection cycle.

Read more: Texas doctor has very personal reason why you should vaccinate your kids »

The symptoms of AFM are similar to those caused by other conditions — notably adenoviruses, enteroviruses, and West Nile virus.

Those affected by acute flaccid myelitis may find they have difficulty controlling their body.

Weakness in the limbs, which can cause limping, is a primary symptom.

People may also experience a lack of facial coordination, including drooping facial muscles and slurred speech. In severe cases, the muscles that control breathing can be weakened, causing shortness of breath.

Typically, children with AFM will show some of these symptoms within a few days of the onset of the disease.

These symptoms are similar to those associated with polio, which has been eradicated in the United States.

Read more: Zika virus: How long will the epidemic last? »

The first step to safeguarding against AFM is to make sure all vaccinations, particularly the poliovirus vaccine, are up-to-date, according to the CDC.

The West Nile virus has been linked to AFM, so it’s important to take steps to prevent mosquito-borne illnesses. To avoid mosquito bites, the use of repellent is advised. A more proactive approach to prevention involves staying indoors at dusk and dawn, as well as eliminating outdoor standing water, a prime breeding ground for mosquitoes.

Finally, one of the best ways to prevent AFM is to practice basic hygiene: properly and frequently wash your hands with soap and water, avoid close contact with people who are sick, and use disinfectant on surfaces that are touched.

The CDC recommends washing your hands before you touch food, after you go to the restroom, before and after you deal with a person who is sick, or a cut or wound.

To diagnose AFM, a doctor will carefully examine the patient’s nervous system. MRI scans can be utilized to pinpoint the affected areas of the nervous system.

Clinicians can also test cerebrospinal fluid for signs of AFM. These tests are most effective when carried out seven to 10 days after the onset of symptoms.