India, Aug. 4 -- 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.

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 look...