We emit a barrage of whiffy chemicals through our pores and in our breath. Some are a sign that we might be getting ill – and could be used to diagnose diseases up to years in advance.

It was obviously nonsense. That was how analytical chemist Perdita Barran reacted when a colleague told her about a Scottish woman who claimed she could smell Parkinson's disease.

"She's probably just smelling old people and recognising symptoms of Parkinson's and making some association," Barran remembers thinking. The woman, a 74-year-old retired nurse called Joy Milne, had approached Barran's colleague Tilo Kunath, a neuroscientist at the University Edinburgh, at an event he was speaking at in 2012.

Milne told Kunath that she had first discovered her ability after noticing her husband, Les, had developed a new musky odour years earlier. He was later diagnosed with Parkinson's disease, a progressive neurodegenerative illness characterised by tremors and other motor symptoms. It was only when Milne attended a group meeting for Parkinson's patients in her home town of Perth, Scotland, that she made the connection: all the patients had the same musky smell.

"So, we then decided to test whether she was right," says Barran, who worked at the University of Edinburgh at the time but is now at the University of Manchester.

It turned out that Milne was no time-waster. Kunath, Barran and colleagues asked Milne to sniff 12 T-shirts, six of which had recently been worn by Parkinson's patients, alongside six worn by others without the disease. She correctly identified the six patients. What's more, she identified one further person who less than a year later was diagnosed with Parkinson's.

"That was kind of amazing," says Barran. "She pre-diagnosed the condition, just like she'd done with her husband."

In 2015, news of her astounding ability made headlines around the world.

Milne's story is not as outlandish as you might assume. People's bodies give off a range of different odours. A new smell may indicate that something has changed, or gone wrong in the body.

Now, scientists are working on techniques for systematically detecting whiffy biomarkers that could speed up diagnoses of a dazzling array of conditions ranging from Parkinson's disease and brain injuries to cancer. The key to spotting them may have been hiding right under our noses.

"It drives me mad that people are dying and we are putting needles up people's butts in order to find out if they have prostate cancer, when the signal is already outside and detectable by dogs," says Andreas Mershin, a physicist and co-founder of RealNose.ai, a company that is developing a robotic nose for diagnosing diseases based on scent. Such technology is necessary as relatively few people have noses powerful enough to detect these tell-tale biochemicals that crop up in the early stages of a disease.

Joy Milne, it turned out, was one of those few. She has hereditary hyperosmia, a trait that means that her sense of smell is much more sensitive than that of the average human – a kind of super-smeller.

There are some diseases that give off such a strong characteristic whiff that most humans can smell them. The breath or skin of people with diabetes who are having a hypoglycaemic episode, for example, can have a fruity or "rotten apples" aroma to it due to the build-up of fruity-smelling acidic chemicals called ketones in the bloodstream. These are produced when the body metabolises fat instead of glucose.

People with liver disease can emit a musty or sulphurous odour from their breath or urine, while if your breath smells of ammonia or has a "fishy" or "urine-like" aroma to it, then this could be a sign of kidney disease.

Some infectious diseases also give off characteristic smells. Sweet-smelling poo could be a sign of infection with cholera or the Clostridioides difficile bacteria, which is a common cause of diarrhoea – although one study found a group of unfortunate hospital nurses were unable to accurately diagnose patients by sniffing their faeces. Tuberculosis, meanwhile, can cause a person's breath to smell foul, like stale beer, and their skin like

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