A paralyzing illness - resembling polio- appears to be spiking in children across U.S. and scientists have no cause and no cure.
It's called acute flaccid myelitis and it is very rare, with the odds of contracting it less than 1 in a million.
So far, AFM has developed in 22 states, with 62 patients diagnosed, and 65 more suspected.
California has no confirmed cases so far, but the State Dept. of Public Health acknowledges four possible cases are being analyzed at the Federal Centers for Disease Control.
"It attacked Julia's body and her spinal cord a certain way," said parent Josh Payne of Minnesota, one of the first states where clusters emerged.
Payne's toddler daughter is hospitalized with a sudden loss of sensation and strength in her limbs.
"It seems to be happening to other kids too," added Payne.
Acute flaccid myelitis typically begins with flu-like symptoms.
It spiked in 2014 and 2016 and seems to surge in late summer and early fall, but much is unknown.
"Despite a lot of investigation by CDC and our partners, AFM remains a mystery disease," said Dr. Nancy Missonnier of the Centers for Disease Control & Prevention.
"Parents should be on the lookout for the sudden onset of weakness in their child's arms or legs and that should prompt them to call their health care provider," said Missonnier.
Over the past four years, slightly fewer than 400 cases have been diagnosed nationally, 90 percent of them under age 18.
"No patient who has been tested by CDC has been positive for polio, which is good," said Dr. Alexander Evens, an infectious disease specialist based in Marin County.
"They presume that it could be viral but they have not found a virus," Evens noted.
West Nile virus can cause paralysis, and some patients are positive for enterovirus, which is associated with the common cold as well as meningitis.
But AFM isn't consistent among patients, which makes it hard to discern who is at risk, and whether they will recover fully or not at all.
"Some patients recover completely, and some haven't," Dr. Evens told KTVU, "and since that hasn't been the same, it suggests it may not be one unifying disease."
At Corte Madera Park Wednesday evening, parents watched their children's soccer practice, and wondered about the mysterious AFM.
"Anything like that is concerning," said dad Oliver Weir, "but I don't know enough about it."
Young players asked some of the same questions scientists are pondering.
"Does it hurt your joints?" queried one boy.
"Is it an illness that happens all your life ?" posed a teammate.
At the nearby skate park, one mom said her son tumbling off his scooter was more worrisome than a rare disease.
"There's not much you can do anyway, other than what you do all the time, like wash your hands, don't share utensils," Rebecca Nessel told KTVU.
As a physician herself, Nessel said she hopes parents focus on the far more common flu- and vaccinate--- rather than stress about AFM.
"It is scary, like a polio-type illness," said Nessel, "but we can't live our lives afraid all the time."
For those working in communicable disease, it is frustrating to watch AFM cases grow as answers remain elusive.
"It is concerning to us because we like to know what's going on," said Dr. Evens, "so we can develop a plan to prevent and a plan to treat, so this is nerve wracking."