No one is certain of exactly what it is or why it’s happening, but Esther and Arthur Herzfeld can be sure of one thing: Something very serious is happening to their four children. Tziporah, Rivka, Tzvi, and Racheli Herzfeld are battling what Fox News is calling a “mystery illness,” that is slowly robbing them all of their ability to walk.

Though they started out as healthy, active children, the Herzfeld kids began to struggle with their walking one by one. Stumbling, then falling, then being unable to get up from falls, each child's condition progressed to a different point of severity. Eighteen-year-old Tzvi can no longer walk at all, and his sisters are forced to “walk slowly and carefully, with the terrifying knowledge that they could fall at any moment, unable to get up on their own,” according to the Record.

“Imagine being completely healthy one day, and then one day you go to sleep, you wake up, you start walking funny and within the next year you can’t walk anymore,” Tzvi told the Record.

While it is odd enough that all four of the Herzfeld children suffer from the same affliction, worse still is that doctors have been unable to identify what that affliction is. When Tziporah, the first of the kids to notice symptoms, was first looked at, a pediatric neurologist diagnosed her with Charcot-Marie Tooth Disease (CMT), a group of hereditary disorders that damage peripheral nerves. CMT is marked by muscle weakness, foot deformities, and loss of balance.

Tests to confirm this diagnosis have all come back inconclusive, and have only contributed to the family’s debt, according to Fox News. Geneticists working with the family have tried the most complex type of genome sequencing, but have not come up with any concrete explaination for the condition.

“It is a little bit concerning that we’ve done all this testing and still cannot really figure out what it is,” Dr. Helio Pedro, section chief of genetics at Hacknsack University Medical Center, said to Fox News. Dr. Pedro has been working with the Herzfelds since August 2014.

The ongoing tests and home renovations to make the house handicap accessible have left the family with at least $100,000 in debt. Friends and family have tried to help out the Herzfelds by creating an online fundraising page, and increased media exposure has led to more donors.

“We’re trying to figure out what this is, and the best way we can [is to] live our lives as normally as possible,” Rivka told Fox News. “But also, there’s a new definition of normal when you have a situation like this, and it’s something that’s hard to swallow, but at the same time we try to do it as best we can with as much grace as we can.”