Silas Waller has measured his life in hospital stays. He has neurofibromatosis, a genetic disorder that causes tumors to form on nerve tissue. The condition makes it hard for him to walk.

“My childhood was a constant string of going to and from hospitals,” said Mr. Waller, 18. “My first year of high school, I had over 50 absences, purely because I had that many doctor’s appointments.”

He was just 6 when tumors first appeared on his spine and damaged his sciatic nerve, which extends from the lower back down each leg. The nerve damage, coupled with scoliosis that was diagnosed, impaired Mr. Waller’s mobility. He developed clubfoot and had a series of operations on his foot.