In 2014, August, AFM first hit the US. Despite the increase, the condition is still considered very rare, occurring in fewer than one in a million people in the US each year.
While aspects of the condition are puzzling, Messacar says it's no mystery disease. Viruses in the enterovirus genus - a group that includes the common cold and the polio virus - are most closely associated with AFM, though past cases have also been linked to West Nile virus, Japanese encephalitis virus, Saint Louis encephalitis, and adenoviruses. It can often infect people with mild flu-like symptoms or no symptoms at all. But in very rare cases, they can damage the nerves.
Experts aren't sure what is causing the increase, but they know the numbers started to go up about four years ago in 2014.
State medical officials released a statement on the outbreak last week so that doctors can be on the lookout for more cases.
Minnesota officials have not yet identified a particular virus in the six cases there.
Since mid-September, six children under the age of 10 were hospitalized and diagnosed with AFM in Minnesota. They come from all over the state, including the Twin Cities area and northeastern and central Minnesota. Symptoms include sudden arm and leg weakness, drooping eyelids, facial weakness, difficulty moving the eyes, slurred speech and difficulty swallowing.
"We don't yet have any effective treatment for the virus or for the condition", Messacar said. Treatment and therapy can restore lost mobility over time, but a loss of the muscular function to breathe can be deadly.
Affected muscles may remain weak, but it's possible to strengthen surrounding muscles so kids regain movement, Messacar said.
A GoFundMe page has been set up to help one of the patients, 7-year-old Quinton Hill. This virus is transmitted from person to person and it can have severe effects of the brain and spinal cord of the system. "There's not much that can be done and as a parent that's very hard to deal with".
"It's always important to practice disease prevention steps, such as staying up-to-date on vaccines, washing your hands, and protecting yourself from mosquito bites", the CDC said on a page devoted to AFM.
However, he said it wouldn't be surprising to see an enterovirus fill an infectious disease niche left by polio, given that polio is also caused by an enterovirus.
EDITOR'S NOTE: This story was written by Maggie Fox, NBC News.