Humans can smell sickness in others because their immune systems are more active, a new study has found.
A team of researchers at the Karolinska Institutet, in Sweden, say there is anecdotal and scientific evidence to suggest that diseases have particular odours.
For example, Scrofula, an infection of the lymph nodes, reportedly smells like stale beer, and a person who suffers from diabetes will sometimes have acetone-smelling breath.
The team, led by Professor Mats J. Olsson, recruited eight healthy participants and injected them with either a form of lipopolysaccharide (LPS) - a toxin made from bacteria and known to ramp up an immune response, or a saline solution.
The participants wore tight t-shirts which absorbed sweat molecules over a four-hour period.
A separate group of 40 participants were then asked to smell the sweat samples from t-shirts. Overall, they rated t-shirts from the LPS group as having a more intense and unpleasant smell than the t-shirts worn by participants injected with saline. They also rated the LPS shirt as having an unhealthier smell.
They concluded that participants who were having a stronger immune response were found to have more unpleasant smelling odours by others.
Prof Olsson said being able to detect these smells could represent a critical adaptation to help humans avoid potentially dangerous illnesses.
"The question we asked ourselves in this study was whether such an adaptation might exist already at an early stage of the disease, thereby reflecting a biomarker for illness," he said.
"Interestingly, in a chemical assay the researchers found no difference in the overall amount of odorous compounds between the LPS and control group.
"This suggests that there must have been a detectable difference in the composition of those compounds instead."
The study is published in the journal Psychological Science.