Even diseases have particular smells. Hard to believe?
If we look into a thrilling research, humans are able to smell sickness in someone whose immune system is highly active within just a few hours of exposure to a toxin.
"There may be early biomarkers for illness in the form of volatile substances coming from the body," explained Mats Olsson of Karolinska Institutet in Sweden.
To test this hypothesis, Olsson and his team injected eight healthy people with either lipopolysaccharide (LPS) - a toxin known to ramp up an immune response - or a saline solution.
The volunteers wore tight t-shirts to absorb sweat over the course of four hours.
Participants injected with LPS did produce a noticeable immune response, as evidenced by elevated body temperatures and increased levels of a group of immune system molecules known as cytokines, said the study published in the journal Psychological Science.
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Now, the researchers asked a separate group to smell the sweat samples from t-shirts.
They rated t-shirts from the LPS group as having a more intense and unpleasant smell than the other t-shirts.
That is, the greater a participant's immune response, the more unpleasant their sweat smelled, the study noted.
"While the precise chemical compounds have yet to be identified, the fact we give off some kind of aversive signal shortly after the immune system has been activated is an important finding," said the researchers.
People with diabetes, for example, are sometimes reported to have breath that smells like rotten apples or acetone.
Being able to detect these smells would represent a critical adaptation that would allow us to avoid potentially dangerous illnesses.