As the decline continued, another neurologist said it was more likely she had Guillain-Barre syndrome. The next diagnosis was chronic inflammatory demyelinating polyneuropathy, an autoimmune disease with similar symptoms but which is reversible. Doctors recommended a spinal tap, and the Williamses were "ecstatic! We thought that with a lot of hard work and physical therapy, Eliza would walk again. And she did, with a walker."

All too briefly.

With some diseases, change is dramatic, and so it was with Eliza. Over a 10-day period her mom and dad watched fearfully as the rigidity in her hips, legs and ankles became more apparent. Everything hurt her, from a diaper change to a trip in the car or being seated in her high chair. Her tremors were coming like tidal waves, her legs shaking. When she sat, she hunched way over and held her arms straight out for balance.

Even with Valium to relax her muscles, her body was stiff. Her toes pointed down like a ballerina in toe shoes.

Back home, a new MRI showed dramatic change in the white matter in Eliza's brain. In September, doctors gave them the diagnosis that would stick: metachromatic leukodystrophy.

They rushed out to get a family photo taken. In it, Eliza is laughing uproariously, cradled on the knees of her parents. She was still very much a 2-year-old at the time. It is the before and after point, because day by day everything would change. This time, they know, she's not coming back.

MLD is one of the leukodystrophies, a set of diseases caused by genetic defects in how myelin produces or metabolizes enzymes. There are different leukodystrophies, each the result of a defect in the gene that controls a specific enzyme. They all impair the growth or development of the myelin sheath, which insulates nerve fibers, according to the National Institute of Neurological Disorders and Stroke. Myelin is made up of those 10 enzymes, and when one of them goes wrong, disease results.

Callahan and Nikki were both unknowing carriers of a genetic defect that is much more common than one would guess. When two carriers have a child, that child may develop the leukodystrophy.

The enzyme that has changed Eliza's future is arylsulfatase A, and the result is toxic buildup of lipids in the cells of the nervous system, liver and kidney. There are three forms: late infantile, juvenile and adult. Eliza has the late infantile form, the most common and most severe.

Typically, children have trouble walking after age 1. Symptoms include muscle wasting and weakness, muscle rigidity, developmental delays, progressive vision loss, convulsions, trouble swallowing, paralysis and dementia. Most children with MLD do not live to age 5. The other forms start later and progress more slowly.