“I’m vegan, so the smell is pretty obvious,” she says. “There’s no other burning flesh going on in the house.”

But it is not only an inability to sense pain that makes Cameron stand out: she also never panics. When a van driver ran her off the road two years ago, she climbed out of her car, which was on its roof in a ditch, and went to comfort the shaking young driver who cut across her. She only noticed her bruises later. She is relentlessly upbeat, and in stress and depression tests she scored zero.

“I knew that I was happy-go-lucky, but it didn’t dawn on me that I was different,” she says. “I thought it was just me. I didn’t know anything strange was going on until I was 65.”

The moment of realisation came when Cameron had X-rays for a bad hip. Now and again her hip would give way, making her walk lopsided. For three or four years, her GP, and then the hospital, turned her away because she was not in pain. When she was finally scanned, the X-rays revealed massive deterioration of the joint. “I’d not had a twinge. They couldn’t believe it.”

Cameron duly had her hip replaced, coping on two paracetamol the day after. But while she was in hospital, doctors noticed that her thumbs were deformed by osteoarthritis. They immediately booked her in for a double hand operation, a procedure described as “excruciating” by one surgeon.

Again, Cameron felt almost no pain after the operation. Consultant Devjit Srivastava, who was overseeing her care at Raigmore hospital in Inverness, was so stunned he referred her to pain specialists at UCL in London.

In a case report published on Thursday in the British Journal of Anaesthesia, the UCL team describe how they delved into Cameron’s DNA to see what makes her so unusual. They found two notable mutations. Together, they suppress pain and anxiety, while boosting happiness and, apparently, forgetfulness and wound healing.

The first is common and reduces activity of a gene called FAAH. The gene makes an enzyme that breaks down anandamide, a chemical in the body central to pain sensation, mood and memory. Anandamide works in a similar way to the active ingredients of cannabis. The less it is broken down, the more its analgesic and other effects are felt.

The second mutation was a missing chunk of DNA that mystified scientists at first. Further analysis showed the “deletion” chopped the front off a nearby, previously unknown gene the scientists named FAAH-OUT. The researchers think this new gene acts as a volume control for the FAAH gene. Disable it with a mutation like Cameron has and FAAH falls silent. The result is anandamide, a natural cannabinoid, builds up in the system. Cameron has twice as much anandamide than most people.

When the researchers explained the mutations to Cameron, a lot of her past made more sense. The time she broke her arm as an eight-year-old and did not tell anyone for days, when the bone started resetting at a funny angle. That she could eat scotch bonnet chillies and feel only a “pleasant glow” in her mouth. That she is always ironing herself and cuts and burns heal so swiftly.

Mutated gene found to cause back problem in almost half of Hongkongers: HKU study

“I was quite amused when I found out,” Cameron said. “Then they told me about these other things, the happiness and the forgetfulness. I’m always forgetting things; I always have done. It’s good in lots of ways but not in others. I don’t get the alarm system everyone else gets.”

Cameron’s mother felt pain normally, as does her daughter. But her son, who carries the second and more important mutation, has a dulled sense of pain. He never takes painkillers and frequently scalds his mouth with hot drinks and food. Scientists suspect that Cameron’s father may have passed the mutation on to her.

James Cox, a researcher on the study, said that in extreme cases, mutations can lead people to feel no pain whatsoever. “This patient doesn’t have a complete loss of pain sensitivity, but we do see that. When they are young, they typically bite off parts of the tongue, and parts of their fingers because they haven’t learned that it’s dangerous.”

China has tweaked genes to create five mad monkeys. Is that ethical?

Cox said of Cameron: “There’s an awful lot we can learn from her. Once we understand how the new gene works, we can think about gene therapies that mimic the effects we see in her. There are millions of people living in pain and we definitely need new analgesics. Patients like this can give us real insights into the pain system.”

Cameron hopes talking about her condition might boost scientific progress. “There may be more like me who are out there that haven’t realised what is different about them,” she says. “If they go and help out with the experiments, it may do something to get people off man-made painkillers and on to more natural ways of relieving pain.”