Every hour or two, John woke up to test the preschooler's blood-sugar level by pricking his finger and squeezing a drop of blood onto a test strip he'd slipped into a meter. A level between 80 and 120 milligrams per deciliter was good. Below that, bad. Potentially very very bad. One night, it was 36. Any lower and Evan could have fallen into a coma. He could have died.

Waking up every two hours to jab your kid's finger is no fun, and after about six months, the Costiks switched over to something called a continuous glucose monitor. Now a tiny sensor implanted just under Evan's skin sent numbers to a pager-like display every five minutes. His parents kept it next to their bed, and while they wished the alarm was a little louder, it sure beat sleeping on the floor.

But children, it turns out, sometimes leave the house. And if those children have continuous glucose monitors, the display unit leaves the house with them. Parents have to rely on teachers, school nurses, and coaches to check the device to see if the kid is headed into blood-sugar meltdown.