Most dog owners have experienced it. You are under the weather, lying on the couch, and your dog will not leave your side. No amount of nudging toward the dog bed convinces them to move. You might joke that your dog knows you feel awful. It turns out that joke is closer to the truth than most people realize.
The question of whether dogs can sense illness has been the subject of serious scientific study for years, and the answers are more compelling than the folklore that inspired them.
The Nose Knows
A dog’s sense of smell is their primary way of experiencing the world, and it operates on a scale that is hard for humans to comprehend. Depending on the breed, dogs have anywhere from 125 million to 300 million olfactory receptors in their noses, compared to the roughly 5 to 6 million that humans have. The part of their brain dedicated to analyzing scent is proportionally about 40 times larger than ours, according to PetMD. That biological advantage helps explain why trained dogs have been able to detect odor changes associated with conditions including certain cancers, diabetes, and epilepsy.
James Walker, former director of the Sensory Research Institute at Florida State University, found that dogs can detect scents at concentrations as low as one to two parts per trillion, making them roughly 10,000 to 100,000 times more sensitive than humans. Alexandra Horowitz, a dog cognition researcher at Barnard College, put it this way in her book Inside of a Dog: while we might notice if our coffee had a teaspoon of sugar added, a dog could detect that same teaspoon in a million gallons of water.
When Your Body Changes
When you get sick, your body chemistry shifts. Pathogens, inflammation, and metabolic changes produce odors that are imperceptible to humans but distinct to dogs. A 2022 study published in PLOS ONE by researchers at Queen’s University Belfast found that trained dogs distinguished combined breath and sweat samples taken before and immediately after a stress-inducing task with 93.75 percent accuracy. Stress-related chemical changes are only one category of shift. Diseases produce their own signatures, and dogs are tuned to notice them.
A 2018 study from the University of Naples Federico II showed that dogs exposed to human sweat collected during fearful experiences displayed more stress behaviors and elevated heart rates, suggesting that emotional and physiological states produce chemical signals dogs detect and respond to.
Medical Detection Dogs, a UK-based charity, has trained dogs to identify specific diseases by scent. Their work shows that trained dogs can detect odor signatures associated with cancers including breast, lung, prostate, and colorectal, as well as diabetes-related blood sugar changes and seizure-related odor patterns. The strongest evidence comes from trained detection dogs, but family pets with no specialized training also respond to illness in their owners.
The Behavioral Shift
Beyond smell, dogs pay close attention to their owners’ routines and behaviors. When someone is ill, their movement patterns change. They move more slowly, sit or lie in different positions, and often deviate from their normal schedule. Dogs are creatures of habit, and they notice when those habits are disrupted.
If you have ever been sick with Hazel around, you know exactly what this looks like. When she senses something is off, she is glued to whoever is not feeling well. It is not her usual “I want attention” paw. It is quieter, more deliberate. She picks her spot and stays.
The University of Naples research suggests even puppies younger than six months respond to human fear-related chemical signals, indicating a biological component to this behavior, not just a learned one.
What Dogs Actually Detect
Dogs can pick up on several categories of illness-related signals. The most scientifically validated involves metabolic changes. When blood sugar rises or falls outside normal ranges, the chemical compounds in breath and sweat change. Dogs notice these shifts and will sometimes alert their owners before any symptoms are consciously felt. This is one basis of diabetes alert dog programs, which have demonstrated measurable detection accuracy in controlled studies.
Stress-related physiological changes represent another category. When cortisol and adrenaline flood the body, they alter the odor profile a person emits, which is likely why dogs react to owners who are anxious or upset, not just physically ill.
Inflammatory responses generate yet another set of signals. Any condition involving inflammation produces metabolic byproducts that differ from healthy baselines. Each disease appears to have its own odor fingerprint. A 2024 study in the Journal of the American Veterinary Medical Association found that two trained dogs detected COVID-19 infections with 94 to 96 percent accuracy in a controlled setting.
Why This Matters
None of this should be surprising. Dogs have spent thousands of years evolving alongside humans, becoming partners in survival. A dog who could sense threats to their human pack had an evolutionary advantage. Those capabilities did not disappear as dogs moved from work animals to family members.
When your dog rests their head on your lap while you are fighting a cold, they are responding to a genuine chemical and behavioral shift that they are biologically equipped to perceive. Their attunement to us is not imagination or projection. It is biology.
The relationship is not one-directional. Your dog is paying attention to you, monitoring your state, and responding to what they observe. The next time your dog refuses to leave your side when you are sick, know that something real is happening. Science is still working out the full picture, but the evidence makes one thing clear: your dog knows more about your body than you probably realize.
If your dog’s behavior shifts and you are not sick, it is worth paying attention to that too. Sudden changes in how your dog acts can mean something, and we wrote about the most common reasons in our piece on why your dog’s behavior suddenly changed. And if you are curious about how your dog’s gut health connects to their immune system and overall wellbeing, including their ability to process the world around them, our guide on gut health and why it affects everything covers that in depth.
This article is for general educational purposes and is not veterinary advice. Always consult your veterinarian if your dog or you are experiencing health concerns.
Tyler Tilton
I do not write surface-level dog advice. When our dog got sick, I learned the hard way that most of what is out there is recycled, watered down, or just wrong. I started The Daily Dog to be the resource I could not find, the one that goes deeper, asks harder questions, and gives you the full picture so you can make real decisions for your dog.
