Have you ever felt like you can smell someone getting sick?
A new study suggests that sickness does actually have a unique odor — that of the overactive immune system. And other humans can actually smell when someone is fighting off an infection.
Not only did sick people smell worse than healthy people, but their stench was even rated as "more unhealthy"— so our bodies seem to know what we are smelling, even if we don't consciously recognize it.
Details of the small study, led by Mats Olsson of Karolinska Institutet in Sweden, were published in Psychological Science on Jan. 22.
Previous research on smells associated with sickness has shown that certain diseases are associated with specific smells. For example, scrofula (a kind of tuberculosis infection) is associated with a stale beer smell and yellow fever smells like a butcher's shop.
But the team from Karolinska Institutet thought smell could be used to identify sickness in a more general way: someone with a ramped up immune system produces a different body odor than someone who is healthy.
To test this idea they recruited eight participants to give body odor samples. During one session each participant was injected with lipopolysaccharide — an endotoxin known to kickstart the immune system in humans. In another session they were injected with saline — a simple salt solution that triggered no immune response.
For both sessions the participants wore tight T-shirts that absorbed sweat easily so a body odor sample could be collected.
The researchers knew the lipopolysaccharide injection successfully activated the participants' immune systems because their body temperature rose and they had elevated levels of cytokines — proteins produced by active immune cells. This is similar to how our bodies react when a virus or bacteria invades us.
Forty volunteers were recruited to take a whiff of the sweaty T-shirts. There were 18 T-shirts to smell total: eight lipopolysaccharide samples, eight saline samples, and two unworn shirts to act as controls. Every volunteer had to smell all the samples twice in a randomized order. They rated the smell of each T-shirt using three criteria:
- Intensity of the smell, rated on a 0 to 7 scale.
- Pleasantness of the smell, rated on a -7 to 7 scale. A rating of 0 meant the smell was neither pleasant or unpleasant.
- "Healthiness" of the smell, rated on a -4 to 4 scale. A rating of 0 meant the smell was neither healthy or unhealthy.
Unsurprisingly, the unworn T-shirts ranked less intense, more pleasant, and more healthy than the lipopolysaccharide or saline T-shirts. You can see how the volunteers rated the lipopolysaccharide and saline shirts in the chart below. The saline ratings are in blue and the lipopolysaccharide ratings are in red.
The researchers think that our ability to detect a difference in the smell of healthy and unhealthy individuals could be a behavioral adaption and a kind of "sickness-cue" that signals we should stay away from the unhealthy person.
This is "the first experimental evidence that disease smells and supports the notion of a 'behavioral immune response' that protects healthy individuals from sick ones by altering patterns of interpersonal contact," the authors write.
The volunteers from the study reported that the most difficult part of the task was assigning a health rating to each smell. The researchers recognize that the health rating could have been influenced by the pleasantness rating — less pleasant smell naturally means less healthy smell, right?
It's probably not that simple. The researchers point to previous research, which suggests that an unpleasant smell triggers disgust and "the emotion of disgust has evolved as a disease-avoidance mechanism."
Interestingly, the volunteers could smell the unhealthiness just a few hours after the immune system was triggered — probably before a person would even know they are infected with a cold or flu. This suggests that smell could be an important warning signal that works to keep us healthy.
SEE ALSO: There Are Only 10 Odor Types That Humans Can Smell