The perceived ability to detect illness in another person through smell has long been treated as anecdotal, yet the concept of a “disease scent” is a recognized area of scientific inquiry. This phenomenon, often dismissed as folklore, is rooted in the body’s complex metabolic response to infection and disease. When the body fights off an invader, like the virus causing a common cold, the resulting biological processes can alter the chemical profile of the air released from the breath and skin. These subtle chemical changes create an olfactory signature that some people claim to perceive. Scientific investigation into these odors is now focusing on identifying the specific compounds that signal a shift in health status, validating the experiences of those who believe their nose can detect sickness.
The Link Between Illness and Body Odor
The human body’s odor is a blend of hundreds of different chemical compounds, which together form an individual’s unique “odor-fingerprint.” When an infection occurs, the immune system is activated, causing metabolic pathways to shift dramatically. This change in metabolism generates different waste products and byproducts than those produced during a healthy state. These new or altered compounds are then released from the body through various routes, including breath, sweat, and skin secretions.
This process is a direct biological consequence of the body’s self-defense mechanisms. For instance, the stress of an infection can lead to increased oxidative stress, where unstable molecules damage cells and lipids. The breakdown products of these cellular processes are volatile and can be expelled through the skin and lungs, subtly changing the person’s overall scent profile. Historically, clinicians have used their sense of smell to assist in diagnosis, recognizing distinct odors associated with certain advanced infections or metabolic conditions.
Volatile Compounds Driving Scent Changes
The specific chemicals responsible for these illness-related odors are known as Volatile Organic Compounds (VOCs). These are carbon-based molecules that easily vaporize at room temperature, making them detectable by the nose. During a respiratory infection, such as a cold, the body’s battle against the virus or bacteria produces unique VOCs from both the host’s altered metabolism and the microbes themselves. Microbial Volatile Organic Compounds, a subset of VOCs, are produced as byproducts of bacterial or fungal growth, and their presence can directly contribute to a distinct scent.
In the context of respiratory illness, researchers examining exhaled breath have identified changes in compounds related to lipid peroxidation, which is the oxidative degradation of fats. For example, compounds like octane and acetaldehyde are products of these stress responses that also occur in milder infections. Acetaldehyde, in particular, is also a known byproduct of certain bacterial metabolisms, suggesting a dual source for this odor signature during a cold. Other classes of compounds, such as alcohols, aliphatic acids, and terpenes, are also frequently found in altered ratios when the body is fighting off an infectious agent.
Why Some Noses Are More Sensitive
The ability to detect these subtle changes in VOC profiles varies significantly from person to person, explaining why only some individuals claim to “smell a cold.” This difference in olfactory perception is often rooted in genetic variation within the human olfactory receptor repertoire. Humans possess several hundred functional olfactory receptor genes, and small differences in the structure of these receptors can make a person highly sensitive to certain molecules while being unable to smell others.
Individuals with heightened sensitivity, sometimes called “super-smellers” or hyperosmics, have specific genetic variations that allow their receptors to bind more effectively to low concentrations of certain odor molecules, including disease-related VOCs. Beyond genetics, acquired sensitivity can develop through repeated exposure or focused attention. Regularly being around sick individuals or intentionally trying to identify subtle odors can heighten a person’s awareness and ability to distinguish these faint scents. The combination of inherited genetic advantage and environmental exposure shapes how acutely a person perceives the chemical signals of illness.
Scent Detection in Other Diseases
The shift in body odor is not exclusive to common respiratory infections; it is a general biological marker that applies to many different diseases. For centuries, physicians have noted the distinctive sweet or “fruity” breath odor associated with diabetic ketoacidosis, a serious complication of diabetes, which is caused by the presence of excess acetone VOCs. The odor profile of a person can change with nearly any condition that significantly alters metabolic processes.
This scientific reality has led to intense research into using odor as a non-invasive diagnostic tool for major diseases. For instance, researchers have identified specific VOCs in the sebum (the oily secretion on the skin) of individuals with Parkinson’s disease, creating a unique musky scent. This discovery was made with the help of a woman with an exceptional sense of smell, leading to the identification of compounds like perillic aldehyde and eicosane as potential biomarkers. Similarly, studies are ongoing to profile the VOC signatures of various cancers, aiming to develop “electronic noses” or breathalyzers capable of detecting disease-specific scents long before symptoms appear.