I just finished reading the last of the Stieg Larsson Millennium series, "The Girl Who Kicked the Hornet's Nest".

In it is a villain who suffers from "congenital analgesia" which means he doesn't feel pain.

It's an exceedingly rare condition. We're talking about less than thirty people in the world! They can't feel pain but they can distinguish between sharp and dull.

The first description of this disorder was in 1932, when a New York physician described a 54-year old man who as a child was struck in the head with a lathing hatchet. He never felt pain as his father removed the tool and a doctor sewed him up.

We now know people with congenital analgesia have both copies of the same mutated gene. That gene, called SCN9A provides instructions for making a certain kind of sodium channel.

Sodium channels are found on all nerve cells, but these particular sodium channels, called NaV1.7, are found only on nerve cells that transmit pain signals to the brain.

Without functioning NaV1.7s, people cannot feel pain whether it's from burns, broken bones or even giving birth. That might sound good, but it's especially dangerous for children. Feeling no pain, they chew on their tongue, break bones and hurt themselves repeatedly

The upside of understanding this disease is that by doing so, scientists hope to develop less risky pain medication. Right now drugs like lidocaine control pain by blocking all sodium channels, even those in the central nervous system.

That means someone given too much lidocaine can end up with seizures and dizziness followed by cardiovascular collapse which can end in death.