The mysterious, rare 'polio-like' disease blighting the US has now spread to 31 states, sickening at least 116 children.

And yet, officials still have no idea what causes acute flaccid myelitis (AFM), nor how to treat or prevent it.

The poorly understood illness, which can cause paralysis and, in rare cases, prove deadly, has struck Colorado the hardest, followed by Texas.

The Centers for Disease Control and Prevention (CDC) is currently investigating a further 170 cases of people with tell-tale symptoms of AFM.

There has already been more than three-and-a-half times as many cases as last year, but doctors remain baffled as to what is causing the illness.

Colorado, with 15 confirmed cases, has been the worst affected state so far this year, followed closely by Texas with 14 people diagnosed with acute flaccid myelitis. A total of 31 states have seen people – mostly children – struck down by the disease, with 19 so far unaffected

The CDC put out its most recent figures on Tuesday, showing there have been 286 reports of people suffering from acute flaccid myelitis (AFM) in 31 states, with 116 of those cases confirmed.

More than 90 percent of people affected by the illness are under 18 and the average age of patients is four.

It is believed to be caused by a combination of viruses and children usually first show signs of a fever and a cough for three to 10 days.

But after the flu-like illness, AFM can suddenly leave people paralysed for life or even end up fatal.

AFM has been called a polio-like illness because of its resemblance to the viral infection, which affected hundreds of thousands of people in the mid-1900s.

It is thought to be unlikely the disease is contagious among people, but it could be caused by viruses which are spread easily.

The CDC last week set up a dedicated task force to try and tackle the illness by investigating its causes and working out how to treat it.

Colorado has been the worst hit state, with 15 cases, followed by Texas with 14.

WHAT IS ACUTE FLACCID MYELITIS (AFM)? The term 'myelitis' means inflammation of the spinal cord. Transverse myelitis is the broad name of the disease, and there are various sub-types. It is a neurological disorder which inflames the spinal cord across its width ('transverse'), destroying the fatty substance that protects nerve cells. That can lead to paralysis. AFM is an unusual sub-type of transverse myelitis. Patients starts with the same spinal inflammation, but their symptoms are different and the disease develops differently. The main distinction is that AFM patients are weak and limp, while patients with general transverse myelitis tend to be rigid. Most AFM patients start to struggle with movement of the limbs, face, tongue, and eyes. They then begin to lose control of one limb or sometimes the whole body - though many maintain control of their sensory, bowel and bladder functions. Unlike transverse myelitis, which has been around for years, doctors are still in the dark about why and how AFM manifests itself. Advertisement

Following those are Washington, Minnesota, Ohio and Pennsylvania, with eight each, Illinois with seven, and New Jersey and Wisconsin with six.

There have been three confirmed cases per state in Arkansas, Alabama, Georgia and Maryland, and two each in Maine, New York City, the Carolinas, Kentucky and Iowa.

And Nevada, Arizona, New Mexico, Montana, Wyoming, Oklahoma, Missouri, Louisiana, Mississippi, Indiana, Virginia and Rhode Island have each had a single case.

The remaining 19 states haven't reported any patients coming down with the disease.

There are likely to be more cases, experts say, and officials are unsure whether the risk is higher in states with more cases, or if they just have better reporting.

Doctors are encouraged to report cases of AFM but they are not required to do so.

Most patients falling ill with the condition are struck down between August and October.

Though the condition remains very rare, CDC director Dr Robert Redfield, who took the job in March this year, said it is the agency's top priority.

Scientists are investigating a number of causes, including viruses, environmental toxins and genetic disorders.

In previous outbreaks, a virus called EV-D68 was implicated in the development of AFM.

'We know that EV-D68 – as well as other enteroviruses – can cause limb weakness, but we don't know what's triggering AFM in these patients,' said the CDC's Dr Nancy Messonnier.

Dr Redfield said in a recent interview: 'CDC's been working very hard on this, since 2014, to try to understand causation and etiology.

'As we sit here today, we don't have understanding of the cause.

'We are, you know, continuing to strengthen our efforts, working in partnership with state and territorial health departments, and academic experts to try to figure this out.'

WHAT IS AFM?

AFM is a rare, but serious condition that affects the nervous system. Specifically it attacks the area of the spinal cord called gray matter, which causes the body's muscles and reflexes to weaken.

Symptoms often develop after a viral infection, such as enterovirus or West Nile virus, but often no clear cause is found.

Patients start off having flu-like symptoms including sneezing and coughing. This slowly turns into muscle weakness, difficulty moving the eyes and then polio-like symptoms including facial drooping and difficulty swallowing.

'If [AFM affects gray matter] lower in the spinal cord [paralysis will] be more in the legs and if it's higher up, it'll be more in the arms,' said Dr Fernando Acosta, a pediatric neurologist at Cook Children's Medical Center, in Fort Worth, Texas.

'Or if it's closer to the neck, they can't move head, neck and shoulders. We had one case of that and that was just awful.'

In the most severe cases, respiratory failure can occur when the muscles that support breathing become weak.

In rare cases, AFM can cause neurological complications that could lead to death.

'It's a pretty dramatic disease; children have a sudden onset of weakness,' said Dr Messonier of the CDC.

No specific treatment is available for AFM and interventions are generally recommended on a case-by-case basis.

Children with weakness in their arms or legs may attend physical or occupational therapy.

The average age of those affected is four years and more than 90 per cent of cases are in children aged 18 and younger. Among them is Julia Payne, two (pictured) from Chicago, Illinois

However, physicians admit they are unaware of the long-term outcomes for those with AFM.

WHO HAS BEEN AFFECTED BY AFM?

The CDC does not track AFM in terms of its prevalence, but rather in outbreaks.

The agency has confirmed 386 cases since an outbreak in Colorado in August 2014, almost all of them in children.

The CDC confirmed 33 AFM cases in 2017, 149 cases in 2016, 22 cases in 2015, and 120 cases in August to December 2014.

While the pattern of AFM most resembles an infectious disease, much remains unknown about the condition.

Among the children infected is two-year-old Julia Payne from Chicago.

She remained in the pediatric intensive care unit at Lurie Children's Hospital for weeks on a respirator and using a feeding tube because she was unable to swallow.

She has since been discharged and transferred to Shirley Ryan AbilityLab, a rehabilitation center where she will face several weeks of physical therapy to regain strength and movement.

In Minnesota, four-year-old Orville Young was likely the earliest confirmed case in the state, according to the Star Tribune.

Orville has been in physical therapy for the last month-and-a-half. His mobility and gait have not returned to normal, but his legs are mostly functional now.

His right arm, thus far, is still paralyzed.

Fortunately many make a full or nearly full recovery of their movement, as did five-year-old Elizabeth Storrie of Willow Park, Texas.

She spent a month at Cook Children's Hospital, in Fort Worth, on IV fluids and a feeding tube until her condition improved.

AFM affects the nervous system and most resembles the polio virus. Health officials have determined it is caused by a virus, but have been unable to pinpoint an exact cause. Orville Young, four (pictured), of Minnesota, was likely the earliest confirmed case in the state

WHAT ARE THE IMPLICATIONS IN THE FIGHT AGAINST POLIO?

AFM has been called a polio-like illness due to its resemblance to the viral infection that impacted hundreds of thousands, particularly between the late 1940s and early 1950s.

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

Poliovirus is not the cause of any of the cases, but some cases have been linked to the enteroviruses EV-A71 and EV-D68, both of which are distant relatives of polio.

Some cases have also been linked to rhinovirus.

'I'm not old enough to have seen a case of polio during my time in practice, but my colleagues who have say [AFM] is similar to what they saw back then,' Dr Acosta said.

'Is this a variant? Potentially, but we don't know.'

In 1957, the US government approved the polio vaccine.

After a nationwide campaign to get children immunized began, the numbers began falling drastically and, in 1979, polio was declared to be eradicated in the US.

This year, Afghanistan and Pakistan are the only two countries where cases of wild poliovirus have been confirmed – largely due to poor sanitation and low levels of vaccination coverage.

However, global eradication is now at risk due to vaccine-derived poliovirus (VDPV) in five countries in Africa this year.

Health experts say this could result in silent transmission of both polio and AFM, because both can lead to paralysis if left undetected.

Anti-vaxxers have blamed childhood polio vaccines for the outbreak, despite physicians saying there is no evidence to suggest this is the case.

'There is no evidence vaccines are causing this,' said Dr Acosta.

'And if we identify the agent that is causing it, the next step would be to develop a vaccine.

'The reason why you see lower rates of polio, whooping cough and other diseases is because we have vaccines that have made them very rare.'

HOW CAN YOU PROTECT YOURSELF?

The CDC advises getting vaccinated against poliovirus and West Nile Virus due to both being potential causes of AFM.

Health experts say this does not simply mean just staying up-to-date with vaccinations, but also minimizing exposure to mosquitoes.

Additionally, you can use warm water and soap to avoid getting sick and spreading germs.

'It's a one-in-million chance to get this so it's extremely unlikely your child will get this,' said Dr Acosta.

'Even if they have sudden onset of weakness, AFM is unlikely to have caused it. It's more likely to be a stroke.

'However, if your child develops it, bring them in and this gives them the best chance of survival.'