Dogs that sense illness in a person typically respond with persistent, focused attention directed at a specific body part or at the person overall. This can look like intense sniffing, prolonged staring, pawing, nudging with the nose, licking a particular spot, or refusing to leave someone’s side. These behaviors stem from a dog’s extraordinary sense of smell, which is at least 1,000 times more sensitive than a human’s, allowing them to pick up on chemical changes in breath, sweat, and skin that we can’t perceive at all.
What makes these behaviors stand out is their persistence and specificity. A dog that smells something medically significant doesn’t just sniff once and walk away. It returns to the same spot, fixates on the same person, and often escalates its attempts to get attention.
What Dogs Are Actually Smelling
Every illness changes your body chemistry. Infections, cancer, blood sugar shifts, and neurological conditions all alter the mix of volatile organic compounds (VOCs) your body releases through your skin, breath, and urine. These are chemical signatures, invisible to us but detectable by a dog’s nose. Dogs have scent membranes inside their noses that can be as large as a handkerchief, compared to a pair of postage-stamp-sized patches in humans. That massive surface area lets them detect molecular changes at concentrations we have no hope of perceiving.
Hypoglycemia in people with type 1 diabetes, for example, triggers the production of volatile compounds that create a fruity or sweet odor on the breath and body. Dogs can pick up on this shift before a person feels any symptoms. Similarly, certain cancers produce distinct VOC profiles in urine and breath, and Parkinson’s disease alters the scent of skin oils. The dog isn’t “sensing” illness in some mystical way. It’s smelling real chemical differences.
Common Behaviors in Untrained Dogs
Most people asking this question don’t have a trained medical alert dog. They have a pet that’s acting strangely. In untrained dogs, illness detection tends to show up as a cluster of unusual behaviors rather than a single clear signal.
The most commonly reported behaviors include:
- Persistent sniffing or licking of a specific body area, often where a tumor or infection is located
- Intense staring at the person, sometimes from across the room, with a fixed and serious expression
- Pawing or nudging at the person’s hand, leg, or torso, as if trying to get their attention
- Clinginess or guarding behavior, such as refusing to leave the person’s side, following them room to room, or sleeping pressed against them when they normally don’t
- Whining, barking, or vocalizing in a way that seems directed at the person rather than at a noise or stimulus
- Reluctance or anxiety, including pacing, restlessness, or a change in their overall demeanor
The 1989 case that first put canine illness detection on the medical radar involved a woman whose dog kept sniffing and nuzzling a mole on her thigh. The dog’s fixation was so unusual and persistent that the woman sought medical evaluation. The mole turned out to be melanoma. That case, published in The Lancet, launched decades of research into what dogs can detect and how reliably they do it.
How Trained Alert Dogs Signal
Trained medical alert dogs behave differently from pets because they’ve been taught specific, repeatable signals. Diabetic alert dogs are commonly trained to bark, jump up, paw at their handler, nudge with their nose, or lick when they detect a blood sugar drop. Labrador Retrievers, one of the most popular breeds for this work, are frequently trained using some combination of all five of these alert behaviors. The goal is always the same: interrupt whatever the person is doing and make the alert impossible to ignore.
In research settings, dogs trained to detect diseases in samples use a “stand-stare” behavior, planting themselves in front of the target sample and holding their nose close to it for five seconds or more. Some dogs sit instead, pressing their nose against the sample. The choice depends on the individual dog’s temperament. Dogs prone to frustration sometimes bite at equipment or vocalize when forced to hold a standing alert, so researchers switch them to a calmer seated signal.
The key difference between a trained alert and a pet’s behavior is consistency. A trained dog performs the same sequence every time. An untrained pet may cycle through several different attention-seeking behaviors, but the common thread is repetition and focus on one person or one body part.
Diseases Dogs Can Detect
The range of conditions dogs have been shown to detect is broader than most people expect.
In cancer research, dogs have demonstrated the ability to identify multiple cancer types from urine samples alone. Detection rates vary by cancer type: 92% for mast cell tumors, 91% for lymphoma, 82% for a blood vessel cancer called hemangiosarcoma, and 73% for melanoma. Overall accuracy across cancer types reaches about 87%. These numbers come from controlled laboratory conditions, not casual pet observations, but they confirm that the chemical signals are real and detectable.
For Parkinson’s disease, trained dogs correctly identified skin swab samples from patients 70% to 80% of the time in a double-blind trial, while correctly ruling out healthy samples 90% to 98% of the time. The subjects with Parkinson’s were not taking any medication, meaning the dogs were detecting the disease itself rather than the scent of drugs. This matters because Parkinson’s can be difficult to diagnose early, and the scent changes may precede obvious symptoms.
COVID-19 detection studies found that dogs trained on samples from infected individuals could identify positive cases with an average sensitivity of about 71% for the Delta variant, though accuracy dropped to around 55% for Omicron. Dogs trained on samples from people without symptoms performed just as well as those trained on samples from symptomatic patients, suggesting the VOC signature exists regardless of whether someone feels sick. Earlier research showed dogs could also identify people carrying malaria from scent alone, even when those individuals had no symptoms.
Diabetic alert dogs respond to both dangerously low and dangerously high blood sugar. The chemical changes are distinct enough that dogs can be trained to give one signal for low blood sugar (such as sustained nose contact with a sample) and a completely different signal for high blood sugar (such as sitting down).
Alert Behavior vs. Normal Attention Seeking
The practical challenge for pet owners is figuring out whether their dog is signaling something medical or just wants dinner. A few patterns can help you tell the difference.
Illness-related behavior tends to be focused on a specific person or body part and doesn’t resolve when you offer food, a walk, or attention. A dog seeking attention will typically settle once it gets what it wants. A dog responding to a scent change keeps returning to the same behavior even after you’ve engaged with it. The behavior also tends to be out of character. If your dog has never been a licker and suddenly won’t stop licking your arm, that’s more notable than if your dog licks everything all the time.
Context matters too. Research on dogs exposed to human stress sweat found that dogs became more anxious themselves, showing stress-related behaviors and becoming less willing to approach. If your dog seems simultaneously drawn to you and distressed, hovering nearby but pacing or whining, that combination is more suggestive of a genuine scent response than simple boredom or separation anxiety. Anxious dogs that are triggered by being left alone typically calm down once you’re present. A dog reacting to an illness-related scent may actually become more agitated when close to you, not less.
None of this means you should treat your dog as a diagnostic tool. But if your dog’s behavior shifts in a way that’s persistent, specific, and resistant to normal explanations, it’s worth paying attention to your own health rather than dismissing the dog as “just being weird.”