Scientists believe they now understand why. In a paper published Thursday in The British Journal of Anaesthesia, researchers attributed Ms. Cameron’s virtually pain-free life to a mutation in a previously unidentified gene. The hope, they say, is that the finding could eventually contribute to the development of a novel pain treatment. They believe this mutation may also be connected to why Ms. Cameron has felt little anxiety or fear throughout her life and why her body heals quickly.

“We’ve never come across a patient like this,” said John Wood, the head of the Molecular Nociception Group at University College London.

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Scientists have been documenting case studies of individuals who experience little or no pain for nearly 100 years. But the genetic mutation that seems to be responsible for Ms. Cameron’s virtual painlessness had not been previously identified.

The study emerged amid major developments in the emotionally charged debate over how to responsibly treat pain. On Thursday, New York State filed one of the most sweeping legal cases yet against the Sackler family, which owns Purdue Pharma, the maker of the opioid OxyContin.

And it was yet another reminder that we need less addictive alternatives for chronic pain, said Dr. Stephen G. Waxman, a neurologist at Yale and the author of “Chasing Men on Fire: The Story of the Search for a Pain Gene.” Dr. Waxman was not involved in the recent paper but he also studies people who have rare mutations that alter their experience of pain.