Humans can tell an unhealthy individual just from their face and smell using a network hardwired into all of us to keep us away from contagion, a new study has shown.
It stands to reason that animals - and particularly social animals like humans - should be able to recognise the danger-signs associated with infection or disease in one another.
Spotting these signals allows an individual to take avoiding action and minimise the chances of succumbing themselves, perhaps by avoiding the source. Indeed, studies have shown that animals use both visual and odour signals as giveaways that one of their number is unwell. But how do humans do this, and how sensitive might we be to the effect?
To find out, Christina Regenbogen, from the Karolinska Institute in Stockholm, Sweden, recruited a group of 18 healthy men and women.
Headshots and odour profiles were collected from the subjects both before and then 2 hours after they received a small injection of a chemical called lipo-polysaccharide (LPS).
This harmlessly and temporarily activates the immune system, producing symptoms mimicking a bacterial infection.
The images and odour profiles from the subjects were then administered to a panel of 30 "raters" cocooned inside an fMRI brain scanner.
Faces from subjects after they were made to feel ill by the LPS injection were scored by the raters as significantly less likeable, less healthy and less socially attractive compared to images of the same individuals before they received the LPS injection.
The raters did not consciously score the smells from the individuals as significantly different between the healthy and unhealthy states, but the brain scans showed that the smell-processing areas of their brains were nonetheless picking up on the change.
Overall, the brain scans from the raterss exposed to healthy-feeling individuals were quite different from the patterns of activity observed when they looked at, and smelled, unhealthy-feeling subjects.
Areas of the nervous system concerned with integration of information were also strongly active, indicating that the brain is drawing together a combination of visual and olfactory (smell) cues from the subjects and using this to influence the rater's perception of the person they are looking at.
The paper, published in PNAS this week, confirms that humans are very sensitive to illness cues and can pick them up within just 2 hours of a potential infection taking hold.
This, the researchers say perhaps shows why "people tend to prefer and be more willing to socialise with healthy people than those who are sick."