The real reason you can’t bear sitting next to some people...

You don’t consciously notice the danger signs: they are too subtle. But your nose and brain do. They constantly sniff for trouble on people around us — in the form of illness and infection — and tell us when to avoid them, for fear of catching their illnesses.

So suggests emerging research by Mats olsson, a professor of neuroscience at the prestigious Karolinska Institute in Sweden.

He has found that our noses tell us instinctively to avoid people who are in the earliest stages of an infection — even before they show any physical signs of illness.

In a study he led, published last year in the journal Proceedings of the National Academy of Sciences, 22 volunteers were injected with harmless bacteria that activated their immune systems as though they had caught a contagious bug.

After a few hours, smell samples were taken from them and given to 30 people to inhale, along with samples from uninjected people. The participants were then asked to rate the volunteers’ smells for likeability, while their brains were monitored by an MRI scanner.

Samples taken from the injected volunteers were rated as far less likeable, although the participants couldn’t describe why.

The brain scans revealed that when the volunteers smelled the infected samples, there was heightened activity in an area called the intraparietal sulcus — involved with filing sensory experiences into our memory — labelling the smells’ owners as people to avoid.

Professor olsson believes that we instinctively do this to avoid catching their bugs. He says: ‘The human brain is actually very good at discovering infection and this motivates avoidance behaviour.’ other research he has done indicates that people’s repulsion to disease odour may stimulate a mild immune reaction, to protect them further against disease.

‘Emotional disgust is there to keep us healthy,’ he adds.

It may also exist to keep us reproducing healthily.

Professor olsson’s research helps to explain why a study by biologists from Tomsk university in Russia, published in the Journal of Sexual Medicine in 2012, found that healthy young women were significantly more likely to rate as ‘unpleasant’ or even ‘putrid’ sweat samples taken from men infected with gonorrhoea than samples from men who were uninfected. B uT what exactly are we sniffing out? A study in March claimed to have the answer: it’s a protein that plays a crucial role in our bodies’ defence systems, called tumour necrosis factor (TNF).

The TNF protein signals to tissues in the body to become inflamed, in order to produce chemicals that fight infections.

u.S. biologists at Wake Forest university in North Carolina have discovered that when TNF prompts inflammation in infected people, it also changes the chemical signature of their body odour, by subtly altering the normal ratios of chemicals that comprise it.

This changed signature may be interpreted by others as an ‘avoid’ sign, to prevent disease spreading, says Dr Patrick Millet, who led the research, published in the journal Brain, Behavior, and Immunity.

He hopes to learn to read these chemical messages in detail as a new way of providing early diagnoses of illness. Meanwhile, a British scientist believes that she can already achieve this, to diagnose Parkinson’s disease long before symptoms start.

Professor Perdita Barran, an analytical chemist at the Manchester Institute of Biotechnology, was inspired by Joy Milne, 67, who made headlines last year by demonstrating that she could discern a distinct odour on people with Parkinson’s.

In the Eighties, Joy complained to her husband, Les, then in his mid30s, that his sweat smelt bad. Ten years later, he was diagnosed with Parkinson’s.

Joy, a retired nurse, only linked the distinctive odour to the disease after meeting people with the same smell at a support group for the charity Parkinson’s uK.

Since then, Professor Barran’s team has collected more than 800 samples of sebum, an oily substance secreted by the skin, from volunteers. In those with Parkinson’s, preliminary tests have found significantly raised levels of several molecules. These could create a unique diagnostic ‘noseprint’ for the disease, says Professor Barran. Her full results are awaiting publication.

It seems we can detect good health from each other, too.

Two years ago, researchers asked 82 women to rate the attractiveness of sweat samples taken from 42 men. The university of Stirling study, published in the journal Appetite, found that samples from men who habitually ate the most garlic were rated the most pleasant, attractive and least intensesmelling.

Why should this be? The study suggested that garlic’s antioxidant and antimicrobial properties may foster low levels of inflammation and this may be transmitted through sweat as a sign of robust wellbeing.