Scientifically Speaking: How seeing sickness activates your immune system

Researchers in Switzerland and Italy found that simply seeing someone who looks contagious can trigger your brain’s threat circuits

You’re in a crowded train. Someone nearby sneezes, eyes watery, nose red. You take a step back. That reaction seems to be social conditioning and a learned experience. But it may run deeper than behavior. Now, we know that there may be more going on, according to a new study in Nature Neuroscience. Responses to visible signs of sickness may not be purely behavioral. They may extend to your immune system too.

People exposed to infectious-looking avatars showed increased activity in innate lymphoid cells. (Representative file photo)

Researchers in Switzerland and Italy found that simply seeing someone who looks contagious can trigger your brain’s threat circuits and mobilize your immune cells. No actual pathogen is needed to trigger a response, just a visual cue. The team tested this using virtual reality (VR), creating digital avatars that looked sick and watching how people’s bodies responded.

This work is fascinating and so simple that one wonders why no one had thought to do it before. In a series of VR trials, 248 healthy participants watched digital avatars approach their faces. Some looked neutral, some fearful, some showed clear signs of illness like rashes or coughing. Participants responded to mild facial touches faster when a “sick” avatar was nearby, even at a distance. Their brains were already on high alert, subtly expanding the monitored space around the body in response to infection threats.

Brain imaging revealed that two key systems kicked in: the peripersonal space network, which monitors the area right around your body, and the salience network, which flags important or threatening information. What surprised scientists most wasn’t just the brain lighting up but what happened in the blood.

People exposed to infectious-looking avatars showed increased activity in innate lymphoid cells. These immune cells act like the body’s first responders, sounding the alarm to other immune defenses. The pattern was strikingly similar to what researchers saw in participants who had received a flu vaccine.

The result is striking and worth repeating: the mere appearance of disease triggered a response normally seen with actual infection.

This suggests your brain and immune system share a biological alarm system, one that’s tuned to err on the side of caution, even when the fire is just a simulation. The researchers traced this remarkable pathway using advanced brain imaging, finding that visual threat detection communicates with immune cells through the hypothalamic-pituitary-adrenal axis. This is your body’s main stress response network, and apparently also a shortcut to priming the immune system before any actual infection.

Using machine learning, the researchers discovered that there is a sophisticated calculation involving stress hormones, inflammatory molecules, and other chemical messengers working in delicate balance. They found that immune activation was predicted to be highest when stress hormones were high, inflammatory factors were low, and certain signaling molecules were in balance.

This makes evolutionary sense. Our ancestors who could spot potential sources of infection from a distance and prepare their bodies accordingly would have had a survival advantage.

But exciting as these observations are, there are limitations. The study didn’t show that this visual immune priming makes you less likely to get sick. It didn’t demonstrate long-term changes in immunity or any boost to actual infection-fighting power. The researchers only looked at two types of immune cells, in healthy young adults, right after exposure. We don’t know how durable the effect is, or whether it works the same across age groups, genders, or backgrounds.

We also can’t separate the sickness cue from disgust entirely. A face covered in lesions might trigger visceral reactions that overlap with fear or revulsion, and that could be part of what’s priming the immune system. The findings are exploratory, the first of their kind, and need replication across different populations and pathogens.

Still, the implications are fascinating. This study could open doors to practical applications such as enhancing vaccine efficacy through visual stimuli or better understanding why stress affects our ability to fight infections. The researchers suggest that one day, something as simple as virtual reality exposure could boost medication effectiveness, though that’s still science fiction, not science fact.

Most intriguingly, this study invites a different kind of question, one biology and medicine don’t always ask: how much of our health is shaped not just by what enters our bodies, but by what we see and expect in the world around us?

Anirban Mahapatra is a scientist and author, most recently of the popular science book, When The Drugs Don’t Work: The Hidden Pandemic That Could End Medicine. The views expressed are personal.