The Centers for Disease Control and Prevention (CDC) says it is investigating 28 new cases across the US of a rare polio-like illness.
That brings the total number of suspected cases to 155, with 62 cases of acute flaccid myelitis (AFM) already confirmed in 22 states, the Daily Mail reports.
The average age of those affected is four years old and more than 90 percent of cases overall are in children under 18, Dr Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases at the CDC, told reporters in a media call on Thursday.
The condition, caused by a viral infection, appears to start off as a common cold, before progressing to paralysis.
But health officials are struggling to determine the specific virus it is linked to or how to treat symptoms aside from waiting them out.
'We have not been able to find the cause of the majority of AFM cases...and we're frustrated that we haven't been able to identify the cause of illness,' Dr Messonnier said.
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,' Dr Fernando Acosta, a pediatric neurologist at Cook Children's Medical Center, in Fort Worth, Texas, told Daily Mail Online in an interview last week.
'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.
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.
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.
Of the 62 cases diagnosed this year, it known that 24 have been in three states: 10 in Illinois, eight in Texas and six in Minnesota.
The states Daily Mail Online is currently aware of with confirmed cases includes: Arizona, Colorado, Georgia, Illinois, Indiana, Iowa, Kentucky, Massachusetts, Minnesota, New Jersey, North Carolina, Ohio, Pennsylvania, Rhode Island, South Carolina, Texas and Washington.
A press officer for the CDC told Daily Mail Online last week that the agency would not be naming the additional states where cases have been confirmed due to 'privacy issues'.
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.
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 even states on its website that symptoms 'have been most similar to complications of infection with certain viruses, including poliovirus, non-polio enteroviruses, adenoviruses, and West Nile virus'.
Poliovirus has been determined to not be 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 that 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. It's the same reason, we developed flu vaccines - to lessen the burden of disease.
'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.'