When a person is ill, a temporary change in body odor, breath, or sweat often occurs. This phenomenon is a direct consequence of the body’s internal response to infection and altered physiological states. The change in scent is a biological signal, driven by the release of specific chemical compounds as the body fights off pathogens. This shift reflects a complex interplay between the immune system and metabolic processes.
How the Immune System Produces Scents
The systemic activation of the immune system is a primary source of the “sick smell” through the production of volatile organic compounds (VOCs). When the body detects a pathogen, it initiates an inflammatory response, generating chemical byproducts released into the bloodstream.
These highly volatile molecules eventually exit the body through exhalation and sweat. Scientific analysis has identified specific compounds elevated during systemic inflammation. For instance, 6-methyl-5-hepten-2-one increases significantly in sweat, modifying the body’s odor profile.
The type of pathogen influences the precise VOCs released, suggesting different illnesses may produce distinct odor signatures. Microorganisms also produce their own volatile compounds as they metabolize nutrients. These microbial VOCs further contribute to the altered scent.
Metabolic Changes and Ketone Release
Changes in eating habits and energy utilization during illness can trigger a distinct odor related to altered metabolism. Sickness often leads to reduced appetite and increased energy demands to combat infection. This combination causes the body to shift away from using glucose and begin breaking down fat stores instead.
The process of fat breakdown, known as ketosis, produces ketone bodies, including acetone. Acetone is a highly volatile substance excreted through the breath and sweat glands. This excretion results in a unique, often fruity smell, sometimes described as similar to nail polish remover.
This metabolic scent is temporary and distinct from inflammation-produced odors, directly linked to the body’s energy deficit. It is a consequence of poor food intake and increased energy expenditure during acute illnesses, signaling the use of an alternative fuel source.
Localized Sources of Odor (Breath, Skin, and Sinus)
Beyond systemic causes, illness symptoms create localized environments that promote odor-causing microbial growth. A frequent source is the breath, often worsened by post-nasal drip from colds or flu. Mucus accumulating in the throat provides an ideal breeding ground for oral bacteria.
These bacteria break down proteins in the mucus, releasing foul-smelling volatile sulfur compounds that contribute to halitosis. Sinus infections also contribute, as trapped, infected mucus in the nasal cavities produces a foul-smelling discharge.
Changes on the skin also affect body odor, especially with a fever. Increased body temperature causes greater perspiration, altering the skin’s natural microbiota. The combination of more sweat and a changed microbial environment allows odor-producing bacteria to flourish, temporarily changing the scent of sweat. Localized bacterial infections, such as strep throat, also release their own waste products and gases, contributing to the overall odor.
Dehydration and Odor Concentration
A final intensifying factor for body odor during sickness is dehydration, common due to fever, reduced fluid intake, or fluid loss. When the body loses more fluid than it takes in, the water available for excretion is reduced. This lack of dilution means the body’s waste products are excreted in a more concentrated form.
The result is a stronger, more pungent smell in both urine and sweat. Urine becomes darker and more concentrated, a classic sign of insufficient hydration. Volatile compounds released through perspiration are less diluted by water, making the body odor more noticeable and intense. Staying hydrated is an important step in minimizing these illness-related odors.