Do you suddenly feel out of sorts with a friendly acquaintance at work and – apart from a weird, stale smell emanating from their body – you don’t know why? Chances are they are about to fall ill with the flu and your brain is advising: stay away.

Swedish researchers have found that the brain detects disease in other people, even before the symptoms break out. It’s able to do this because people who have become infected with illness very quickly produce aversive odours in their skin, breath, blood and urine.

They also, in this short time-frame, develop the look of being sick; observers can see the sickness brewing, but may not consciously know it.

When a healthy person is exposed to these odours and the visual cues – the signals of disease – the immune system kicks in and prepares for an invasion of pathogens.

More than that, healthy people will automatically and subconsciously distance themselves from the newly sick – that is, from people who don’t even know that they’re sick yet – because the immune system prompts us to like infected people less than healthy ones.

Professor Mats Olsson, of the Karolinska Institutet’s Department of Clinical Neuroscience, leads a team of researchers who investigate how exposure to body odour affects our mood, behaviour, and physiology.

In 2014, Prof Olsson and company published a paper The scent of disease: Human body odour contains an early chemosensory cue of sickness. It found that different diseases give off unique odours.

People in the early stages of diabetes, for example, have a form of acetone on their breath – and they smell like over-ripe fruit or fingernail polish remover.

Prof Olsson’s latest paper, Behavioral and neural correlates to multisensory detection of sick humans, published last month in the Proceedings of the National Academy of Sciences, investigates how the immune system – and healthy people – respond to these different smells.

Prof Olsson’s thinking was this: the human immune system does a pretty good job at fighting off infection, but obviously doesn’t enjoy 100 per cent success in doing so; also, resisting sickness costs the body a lot of energy. So avoiding illness in the first place should be part of our survival instinct and strategy.

This new study, he says, proves this to be the case.

“The human brain is better than previously thought at discovering early-stage disease in others,’’ he says, in a prepared statement. “Moreover, we also have a tendency to act upon the signals by liking infected people less than healthy ones.’’

Smelling sickness can actually activate a caring response in your loved ones. Photo: Getty

Olsson’s experiments weren’t much fun for test subjects who were injected with a harmless form of bacteria that, for a few hours, activated the classic symptoms of disease – tiredness, pain and fever.

A second group of subjects were then exposed to these smells and images as well as those of healthy controls, and asked to rate how much they liked the people, while their brain activities were measured in an MR scanner.

They were asked to rate which of the participants looked sick, those they considered attractive and those they might consider socialising with.

And the results? There was, says Professor Olsson, “a significant difference’’ in how people preferring to socialise with the healthy control subjects than those who were temporarily sick and whose immune system was artificially activated.

However, the avoidance response to disease, he says, “does not necessarily apply if you have a close relationship with the person who is ill.”.

In fact, 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,’’ says Olsson.

Scents to smell for

• People who have contracted influenza give off a stale smell.
• The smell of over-ripe fruit or nail polish remover on your breath is a warning sign of diabetes.
• A friend smelling of stale beer may not just be hungover, but infected with scrofula, a glandular form of tuberculosis.
• Like the smell of baked bread? It could mean your mate’s come down with typhoid.
• Who wants to smell like a butcher’s shop? Nobody, because it might mean there’s yellow fever in the family.