You can smell when someone's sick—here's how (2024)

I’m sick, and I don’t smell right. I don’t mean that my nose isn’t working—though this cold has me stuffed up. Instead, my own body odor seems somehow different, sour and unfamiliar.

I’m far from the first person to notice this nasty side effect. Scientists have found that dozens of illnesses have a particular smell: Diabetes can make your urine smell like rotten apples, and typhoid turns body odor into the smell of baked bread. Worse, yellow fever apparently makes your skin smell like a butcher’s shop, if you can imagine that.

It’s curious, but not merely a curiosity; our noses and brains are attuned to these smells, which in turn signal our finely tuned sense of disgust to feel grossed out and thus help us avoid something that could make us sick.

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We might even be able to harness this “sickness sensing” ability. Scientists think that if we could identify the specific chemicals that make up sick smells, we might sniff out diseases that are otherwise difficult to detect early, like cancers or brain injuries.

Some people already have a surprising ability as disease detectors, like a Scottish woman named Joy Milne who has an uncanny ability to smell whether someone has Parkinson’s disease. If we could replicate that skill, perhaps annual physical exams of the future could include a sniff test for various diseases.

The smell of our bodies

We marvel at such a skill, but anyone with working olfactory senses could probably learn to recognize various “sick smells.” Humans are very good at detecting illness, says Valerie Curtis, a public health researcher at the London School of Hygiene and Tropical Medicine and author of the book Don’t Look, Don’t Touch, Don’t Eat on the science of disgust.

“Signs of sickness are some of the things people find most disgusting,” Curtis says—think mucus, vomit, or pus. Disgust is our way of avoiding things that could harm us, so “it simply makes good evolutionary sense that we use our noses to notice illness.” (Of course, people sometimes like stinky things, too.)

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But why would sick people smell differently in the first place? The key is that our bodies are constantly launching volatile substances into the air. They’re carried in our breath and literally ooze from every pore, and they can vary depending on age, diet, and whether an illness has thrown off some cog in our metabolic machinery. Microbes living in our guts and on our skin also contribute to our signature scent, by breaking down our metabolic by-products into smellier ones.

Basically, you’re a walking factory of smells. And if you start paying attention to them, you might notice when something’s off.

Sniffing out Parkinson’s disease

In recent years, the case of the woman who can smell Parkinson’s brought attention to the idea of sniffing for disease. Parkinson’s is notoriously tricky to diagnose; by the time most people learn they have it, they’ve already lost half of the dopamine-producing brain cells the disease attacks. But about six years before her husband Les was diagnosed, Joy Milne noticed that he smelled odd.

Les had a “sort of woody, musky odor,” Milne told the Telegraph in 2019. Years later, in a room full of Parkinson’s patients, she realized the smell wasn’t unique to Les. All the people with Parkinson’s smelled that way.

She mentioned it to a Parkinson’s researcher in Edinburgh named Tilo Kunath, who mentioned it to his colleague, analytical chemist Perdita Barran. They decided the well-meaning Mrs. Milne may have just noticed the characteristic smell of old people; “We talked ourselves out of it,” Barran says.

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That could have been the end of it. But another biochemist encouraged the pair to track Milne down and try a blind T-shirt test: She sniffed six sweaty tees from people diagnosed with Parkinson’s, and six from healthy controls. Milne correctly identified which six had Parkinson’s, but she also tagged one of the control subjects as having the disease.

Despite that error, Barran was intrigued—all the more so eight months later, when the same supposedly healthy control subject Milne had identified was diagnosed with Parkinson’s.

Passing the smell test

The T-shirt test was intriguing, but we have to take it with a scientific grain of salt. After all, there are lots of reasons people might share an odor. Plus, it wouldn’t exactly be practical for everyone who wants to know if they have Parkinson’s to have Mrs. Milne over to smell their T-shirts.

In one notorious dead end, researchers were convinced there was a smell linked to schizophrenia, and a particular compound called TMHA—said to smell like a goat—was identified and described in the prestigious journal Science. There was hope this chemical might even be the cause of schizophrenia, which would open up new avenues for treatment.

But in years of follow-up testing, the results couldn’t be repeated. The TMHA “schizotoxin” went the way of tabletop nuclear fusion—meaning nowhere.

Barran is now the director of a research center at the Manchester Institute of Biotechnology, where she has used the painstaking methods of chemistry to determine which molecules make up the smell of Parkinson’s. She even launched a company in 2021 called SebOMIX that aims to commercialize a test that could detect Parkinson’s disease before a person has even begun to have symptoms. The test consists of a simple skin swab that captures the signature molecules of Parkinson’s in sebum, an oily substance secreted by skin.

First, the team is working to chemically identify molecules that are diagnostic for Parkinson’s, which is harder than it looks on CSI. Of the thousands of known volatile compounds, many are not well characterized or data on them exists only within the fragrance industry.

With funding from Parkinson’s UK and the Michael J. Fox Foundation, in 2019 Barran’s team found three molecules that are elevated in people with Parkinson’s disease (eicosane, hippuric acid, and octadecanal) and one that’s lower (perillic aldehyde). The researchers then blended these molecules to create a chemical fingerprint for the disease. When they presented their signature scent to Joy Milne, she confirmed that it smells like Parkinson’s disease to her, too.

Next, the team had to confirm that these molecules are reliably elevated in Parkinson’s patients, and figure out whether they can detect the smell before symptoms appear. Ideally, they would also learn how Parkinson’s triggers the body to produce the molecules.

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In 2022, Barran’s team reported that under laboratory conditions, their test could detect more than 4,000 unique molecules in sebum, of which 500 varied between people with Parkinson’s and those who don’t have the disease. Overall, the test was 95 percent accurate in determining whether a person had Parkinson’s. Now, the team plans to try out the test in real-world conditions, starting within the national health care system in the United Kingdom and eventually producing a commercial test available for use worldwide.

Barran says she’s up for the challenge—even though her own sense of smell was damaged in an accident and she can’t smell the Parkinson’s odor herself.

“Joy [Milne] has an extremely good sense of smell,” Barran says, “but she isn’t the only person who can smell it. What’s special is how persistent she was in her conviction that it was something that could be used.”

Scent of a patient

That brings us back to the question of what you and I can actually smell. While dogs have the most lauded sense of smell and have been tapped to sniff out cancer, research suggests that humans are just as good at detecting many odors.

Judging by the number of neurons in our brains’ olfactory bulbs, people may be better smellers than rats and mice and fall about in the middle of the pack among mammals. Perhaps the biggest barrier to our abilities is that we don’t pay enough attention to smells, and we lack a sophisticated language to describe them.

“We’re less able to rationalize smell,” says Curtis. She recalls using a soap she had brought home from India: “The idea of ‘India’ popped into my head long before I realized it was the smell.”

Likewise, we may not realize when we’re smelling a change in our own or a loved one’s health.

There are hints, though, that we may be decent illness detectors if we pay attention to the task. In a small double-blind study published in the Proceedings of the National Academy of Sciences in 2017, participants could identify sick versus healthy people based on body odor and photographs just a few hours after some of the people’s immune systems were triggered by a toxin that mimicked infection.

So while we don’t yet have a breathalyzer for disease, we might do well to follow our noses.