"They came across him in the market, sticking daggers through his arms," said John Wood, a professor at University College London in England and a member of the research team.
From talking to the locals the researchers were able to find six other people from three families in the same region who also had the disease.
These people were able to feel hot and cold, sense pressure, and could tell sharp objects touching their skin from blunt ones.
They were all deemed to be of at least average intelligence. But none of them felt any pain.
Surprisingly, the families were unrelated, but all of the afflicted members had mutations to the same gene (get an overview of human genetics).
The culprit gene encodes a protein that forms part of a channel that allows electrically charged particles to cross the surface of nerve cells.
"This channel is undoubtedly crucial for human pain perception," Wood said.
The mutation stopped the protein from working, the study showed. This apparently prevented a certain type of nerve cell from passing on signals that tell the brain when something is painful.
Learning by Example
Early childhood can be treacherous for people with complete insensitivity to pain.
They can gnaw endlessly on their tongues and fingers during teething, stick their fingers in their eyes, or suffer major injuries without noticing.
The six people studied for the Nature paper all had permanent injuries to their lips or tongues from biting themselves when they were young.
The boy who clued researchers to their presence in Pakistan died after jumping off a house before his fourteenth birthday.
But eventually sufferers can learn what to avoid doing even without pain as a guide.
Paola Sandroni, a neurologist at the Mayo Clinic in Rochester, Minnesota, who was not a member of the research team, had a patient with a nearly identical disorder to the Pakistani subjects.
This patient was very unusual, because he was 20 years old before he was diagnosed with a nerve disorder.
When he was a toddler he complained to his mother that some kids had roughed him up and hurt his feelings. What he didn't mention was that he also had broken his arm.
But eventually, Sandroni said, "he learned by using his intellect and experience of watching others."
He once shocked himself while working on some wiring, for example.
"His arm started jumping, and he felt nothing," Sandroni related.
"He thought it was pretty funny, actually Then he said, Oh, maybe I shouldn't be touching these wires. So he pulled his hand away and noticed that he had a burn."
When Sandroni reported the diagnosis earlier this year, she couldn't find reports of any other patients with the set of symptoms.
But, she says, the new cases in Pakistan sound very similar.
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