Sick people often do smell different, and the reason is chemical. Illness changes what the body makes, and some of what it makes floats into the air. The change ranges from a faint shift in sweat during a cold to sharp, recognizable smells that point to specific serious conditions. Doctors have sniffed for clues to disease for thousands of years, and today machines and trained dogs are being tested on the same idea.1Netherlands Journal of Medicine. Smelling the diagnosis: a review on the use of scent in diagnosing disease
How Illness Changes the Chemicals Your Body Gives Off
Your body is always releasing hundreds of airborne chemicals through breath, sweat, skin and urine.2PubMed. The scent of disease: volatile organic compounds of the human body related to disease and disorder The mix mirrors your metabolism, the chemical work of turning food into energy and repairing tissue.3PubMed. The scent of disease: volatile organic compounds of the human body related to disease and disorder When an infection or a metabolic disease changes that work, body odor often changes too.4PubMed. The scent of disease: volatile organic compounds of the human body related to disease and disorder
Skin bacteria amplify the change. Fresh sweat has little smell of its own; bacteria turn its contents into odor. Hormones, food and metabolic shifts change what sweat contains, and a change in skin bacteria or a skin infection can alter sweat chemistry enough to create a distinctive odor.5PubMed Central. Microbiota and Malodor—Etiology and Management
A sick person may also smell worse for ordinary reasons. Fever brings heavier sweating, which gives skin bacteria more to work with. Someone who is bedbound often showers and changes clothes less. Not eating or drinking, or breathing through the mouth, dries the mouth and makes breath stale. None of these is a disease signal on its own. A recognizable smell narrows the possibilities but doesn’t diagnose anything, and food, medicines and hygiene can all mimic illness smells.
Your Immune System Can Change Your Scent Within Hours
When healthy volunteers received a harmless bacterial substance that switches on the immune system, their body odor turned more unpleasant within a few hours.6PubMed. The scent of disease: human body odor contains an early chemosensory cue of sickness The stronger a person’s immune reaction, the bigger the change in their smell.7PubMed. The scent of disease: human body odor contains an early chemosensory cue of sickness
Real respiratory infections give off a faint signal too. People asked to sniff armpit odor from someone with a cold or other respiratory infection and from a healthy person picked the sick one about 57% of the time.8PubMed Central. Humans can detect axillary odor cues of an acute respiratory infection in others Guessing would be right 50% of the time, so the signal is real but weak.9PubMed Central. Humans can detect axillary odor cues of an acute respiratory infection in others Your nose can’t reliably tell you whether someone is contagious.
Fruity or Sweet Breath and Diabetes
When the body lacks insulin, it burns fat for fuel and produces ketones, acids that can build up in the blood. A dangerous buildup is called diabetic ketoacidosis, or DKA, a term you may hear from doctors. One ketone, acetone, forms naturally from another and leaves through the lungs, which gives breath a sweet, fruity smell.10Diabetes/Metabolism Research and Reviews. Ketone bodies: a review of physiology, pathophysiology and application of monitoring to diabetes
Other signs turn fruity breath into an emergency: extreme thirst and frequent urination; nausea, vomiting or stomach pain; fast, deep breathing; confusion or unusual sleepiness. With these, seek emergency care right away.
Not every sweet breath is DKA. Low-carb or keto diets and fasting produce mild ketones that can smell faintly fruity, and that is usually not dangerous in people without diabetes. Someone with diabetes and fruity breath should check ketones with a home test strip if they have one. DKA can also be the first sign of undiagnosed diabetes, so unexplained weight loss plus thirst and fruity breath warrants prompt testing.
Ammonia, Musty and Sulfur Breath from Kidney and Liver Failure
Healthy kidneys remove urea, a waste product, from the blood. When the kidneys fail, urea builds up, and ammonia with it.11PubMed. Recent analytical approaches to detect exhaled breath ammonia with special reference to renal patients Ammonia passes from the blood through the lungs, giving breath an ammonia or fishy smell.12PubMed. Recent analytical approaches to detect exhaled breath ammonia with special reference to renal patients Mouth bacteria breaking down urea in saliva add to it.13PubMed Central. Effects of end-stage renal disease and dialysis modalities on blood ammonia level Dialysis, which cleans the blood, sharply cuts the ammonia in breath.14PubMed Central. Effects of end-stage renal disease and dialysis modalities on blood ammonia level
Advanced liver disease gives breath a different odor. Doctors call it fetor hepaticus, a term that may appear in medical notes. It comes mainly from dimethyl sulfide, a sulfur chemical, with some help from ketones.15PubMed. GC-MS analysis of breath odor compounds in liver patients Other sulfur chemicals are linked to the breath of people with cirrhosis.16PubMed Central. Liver Impairment—The Potential Application of Volatile Organic Compounds in Hepatology
Liver breath is described as musty, sweetish, or like rotten eggs or garlic, while kidney breath may come with a metallic taste. Both usually appear late, alongside other clear symptoms. With kidney failure these include swelling, tiredness, little urine, itching and nausea. With liver failure they include yellow skin or eyes, a swollen belly, confusion and easy bruising. A new smell plus these symptoms needs prompt medical care.
These odors differ from ordinary bad breath. That kind mostly comes from bacteria on the tongue and between the teeth, and it improves with brushing, tongue cleaning and flossing. Odor that comes from the blood is carried out on the breath from the lungs, so it persists even after a careful mouth cleaning. If a smell keeps returning right after brushing in someone who already has kidney or liver disease, call the care team rather than reaching for a stronger mouthwash.
Fishy Body Odor from a Genetic Condition
Trimethylaminuria, also called fish odor syndrome, makes people give off a smell like rotting fish.17The Journal of Clinical and Aesthetic Dermatology. A Review of Trimethylaminuria (Fish Odor Syndrome) Gut bacteria turn nutrients from food, like choline and carnitine, into trimethylamine, a fishy-smelling chemical.18JAMA Dermatology. Trimethylaminuria (Fish-Odor Syndrome): A Case Report Normally the liver converts it into an odorless form.19PubMed Central. Trimethylaminuria: causes and diagnosis of a socially distressing condition In the inherited form, a faulty gene leaves that liver enzyme weak, and the chemical leaves through sweat, breath and urine.20JAMA Dermatology. Trimethylaminuria (Fish-Odor Syndrome): A Case Report
The smell can come and go. It may be stronger after meals rich in choline or carnitine, since those foods give gut bacteria more raw material. Stress, sweating and hormonal changes can intensify it at some times, and some women notice it around their periods.
Doctors can confirm the condition with a urine test. Limiting foods high in choline, such as eggs, liver and some legumes, often helps, ideally with guidance from a doctor or dietitian since choline is an essential nutrient. Slightly acidic soaps and frequent washing can reduce odor. Support for the anxiety and isolation that come with it matters too.
Infections That Have Their Own Smell
Pseudomonas, a bacterium that infects burns and wounds, often gives off a grapelike smell.21PubMed Central. Use of 2-aminoacetophenone production in identification of Pseudomonas aeruginosa That smell is strong on lab plates but has been found only in trace amounts in patients.22PubMed Central. Detection of Pseudomonas aeruginosa in sputum headspace through volatile organic compound analysis A wound’s smell is a reason to get it checked, not proof of a specific germ. Many different bacteria, including ones living without oxygen in deeper tissue, can make a wound smell foul. Clinicians look at the wound’s appearance, drainage, surrounding redness, warmth, pain and whether the person has a fever. If a lab needs to know which germ is involved, it takes a swab or tissue sample and grows it, rather than relying on the nose.
Gut infections change stool odor chemicals. Stool from people with C. difficile, a bacterium that causes severe diarrhea often after antibiotics, contains a distinctive set of them.23PubMed Central. Faecal volatile biomarkers of Clostridium difficile infection One is p-cresol, which has a strong, barnyard-like smell.24PubMed. Development of a novel method for detection of Clostridium difficile using HS-SPME-GC-MS
A wound that smells foul, oozes, or is red and hot may be infected. Severe watery diarrhea with an unusual smell after antibiotics needs a doctor. Fever with either makes it more urgent.
Can Dogs and Machines Smell Disease
Dogs can detect odors at concentrations far lower than current electronic noses can.25PubMed Central. Canine olfactory detection and its relevance to medical detection In one study, trained dogs detected samples containing just 20 ovarian cancer cells.26PubMed Central. Remote Medical Scent Detection of Cancer and Infectious Diseases With Dogs and Rats: A Systematic Review
Diseases without an obvious smell still leave chemical traces. Cancer cells cause chemical stress that breaks down fats into chemicals that can show up in breath.27PubMed Central. Lipid Peroxidation Produces a Diverse Mixture of Saturated and Unsaturated Aldehydes in Exhaled Breath That Can Serve as Biomarkers of Lung Cancer—A Review Skin oil from people with Parkinson’s has a different chemical profile than skin oil from healthy people.28PubMed Central. Discovery of Volatile Biomarkers of Parkinson’s Disease from Sebum That difference held in people from different countries.29PubMed Central. Classification of Parkinson’s disease and isolated REM sleep behaviour disorder: delineating progression markers from the sebum volatilome
The Parkinson’s finding began with a woman who noticed her husband’s smell had changed years before his diagnosis. Researchers then traced the change to skin oil chemicals. It shows a human nose can occasionally pick up a real disease signal.
Electronic noses use sensor arrays to recognize patterns in breath chemicals.30PubMed Central. Advances in Electronic-Nose Technologies for the Detection of Volatile Biomarker Metabolites in the Human Breath In one study, a breath test correctly sorted people with and without COVID-19 about 72% to 82% of the time.31PubMed. Machine learning enabled detection of COVID-19 pneumonia using exhaled breath analysis: a proof-of-concept study But these sensors can be unstable and give false readings.32PubMed. Prospects and Challenges of Volatile Organic Compound Sensors in Human Healthcare You can’t yet replace a blood test or scan with a breath test.
Dogs need long training, and results vary between dogs. Studies are usually small and controlled, and the dogs aren’t available as a routine test for most people.
When a New Body Odor Means You Should See a Doctor
Rule out common, harmless causes first. Garlic, onion, spices, alcohol and fish change sweat and breath. Some medicines and supplements change body odor. Gum disease and tonsil stones cause bad breath, and hormonal changes such as puberty, pregnancy or menopause alter sweat.
A smell that lasts is different. Many harmless causes come and go within days, so a smell that lasts for weeks despite good hygiene and an unchanged diet is more likely to have a medical cause. Some conditions, like the genetic fishy-odor condition or early kidney or liver trouble, can be present before other symptoms are obvious. When the smell is persistent but the person otherwise feels well, a routine appointment, not the emergency room, is the right level of response.
At the appointment, say what the smell is like: fruity, ammonia, fishy, musty or sulfur. Note whether it’s in the breath, sweat or urine, when it started, and any foods or medicines that might be involved. List any other symptoms alongside it. A doctor may check blood sugar and ketones for fruity breath, kidney and liver blood tests for ammonia or musty breath, and a urine test for a persistent fishy odor.