Most people are familiar with the concept and perhaps also the sight of police dogs patrolling airports and other realms of mass transportation for narcotics and explosives. We know that bloodhounds are brilliant at catching the scent of escaped convicts and that even humble mutts can be amazingly quick at finding lost balls in thick underbrush, just by the smell.

But who knew that dogs, when properly trained, can alert a diabetic boy to a dangerous drop in his blood sugar 20 minutes before it happens? Or that a dog trained...