Whether it’s the person brandishing a tissue on the train or the child with the telltale signs of chickenpox, the threat of an infection can be enough for us to beat a retreat.

Now researchers using virtual reality avatars have shown that the mere sight of a potentially contagious person is enough to mobilise our immune system too.

“Although surprising, our finding that immune responses can be triggered by simulated infections presented in VR is consistent with the principle of the smoke detector in biological systems,” the authors said, adding the behavioural system that helps us avoid catching diseases is “exquisitely sensitive” to cues that may suggest someone is sick.

Writing in the journal Nature Neuroscience, the researchers reported how they fitted 248 healthy people with VR headsets and carried out five experiments, each involving a minimum of 32 participants.

In each experiment, participants initially watched three faces of the same sex as themselves repeatedly loom towards them, bearing a neutral expression.

Participants were then split into groups and shown the same three faces multiple times, either with a neutral expression or signs of viral infections such as skin rashes. In some experiments, an additional subset of participants were shown the faces displaying an expression of fear.

In one experiment, participants were asked to press a button as fast as possible after receiving a mild touch to their face while an avatar was shown.

The team found that when the avatars showed signs of sickness, participants pressed the button when the faces appeared further away than for faces showing a neutral or fearful expression.

Results from EEG tests to investigate the electrical activity of the brain tallied with these findings. As expected, as avatars loomed closer, the brain system that represents the space closely surrounding our body became activated. However, this activation differed when the avatars showed signs of infection compared with neutral expressions, even when they appeared far away. These differences, the team add, were localised in areas of the brain involved in detecting and filtering threats.

The team found that functional MRI (fMRI) brain scans supported these results, further revealing that when infectious avatars were shown there was a greater connection between this threat-detection network and part of the brain called the hypothalamus, which acts as a key regulation centre for the body.

The team also found differences in participants’ blood when they were shown the infectious avatars compared with neutral or fearful faces.

“[In terms of cells], we saw mainly that there is an activation of an immune cell family called the innate lymphoid cells (ILCs)that [are] early responders in immunity to basically alarm other immune cells,” said Prof Camilla Jandus of the University of Geneva and an author of the study.

The team added that they found a similar activation of ILCs when they examined the blood of individuals who had received an influenza vaccine but had not been exposed to the VR setup.

Dr Esther Diekhof of the University of Hamburg, who was not involved in the work, said the study chimed with previous research, including that of her own team.

“The study provides yet another good example for the existence of a mechanism that responds to potential contagion threats even before the immune system has come into contact with pathogens,” she said.

But Prof Benedict Seddon, of University College London, said questions remained, including whether the observed responses help the immune system actually fight an infection.

“When we get infected, by Sars-CoV for instance, it can take a day or two for the infection to establish and for the immune system to become aware of it and respond, a long time after the initial encounter that stimulated this short-lived mobilisation,” he said.