We know that animals can sniff out sickness in humans, but humans might be able to detect disease in each other too.
A new study led by neuroscientists from Sweden's Karolinska Institute shows that we can see and smell sickness in others often before there is any sign of it.
![Being unwell can change the smell of our sweat.](http://web.archive.org./web/20170602042721im_/https://www.fairfaxstatic.com.au/content/dam/images/g/w/g/3/z/z/image.related.articleLeadwide.620x349.gwg3g8.png/1496153661055.jpg)
"Throughout human evolution, infectious diseases have been a primary cause of death. Detection of subtle cues indicating sickness and avoidance of sick conspecifics would therefore be an adaptive way of coping with an environment fraught with pathogens," the authors write in the journal Proceedings of the National Academy of Sciences of the United States of America.
In the double-blind placebo-controlled study, 22 participants allowed themselves to be injected with endotoxin (which is harmless but brings on an immune response resulting in pain, lethargy and fever).
Researchers took facial photographs and body odour samples of the participants while "sick" and again when they were "healthy" (having been injected with a placebo saline solution). These photos and samples were presented to another group of 30 participants, who rated each person while undergoing brain scans.
"Faces were less socially desirable when sick, and sick body odours tended to lower liking of the faces. Sickness status presented by odour and facial photograph resulted in increased neural activation of odour- and face-perception networks, respectively," the paper's authors said.
Previous research has found that being shown pictures of very unwell people can prompt an immune response.
"It makes evolutionary sense that the immune system would respond aggressively only when it's really needed," said Mark Schaller, a psychologist and co-author of the study. "If I see a bunch of sick people, maybe a big infection is around, and I better kick my immune system into high gear."
Separate research has also shown that sickness has a strong smell to it.
Diabetes has been described as smelling like rotten apples, caused by low concentrations of acetone on the breath, while kidney disease is associated with an odour of ammonia.
Those with gonorrhea can have sweat that smells "putrid", while glandular disease scrofula is said to leave people smelling like stale beer. People with metabolic disorder phenylketonuria (or PKU) tend to smell musty.
The new research shows that we're even better at detecting disease than previously thought.
"The study shows us that the human brain is actually very good at discovering this and that this discovery motivates avoidance behaviour," said the lead author, Professor Mats Olsson
"Common sense tells us that there should be a basic behavioural repertoire that assists the immune system. Avoidance, however, does not necessarily apply if you have a close relationship with the person who is ill.
"For instance, there are few people other than your children who you'd kiss when they have a runny nose. In other words, a disease signal can enhance caring behaviour in close relationships. With this study, we demonstrate that the brain is more sensitive to those signals than we once thought."
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