Humans have a natural aversion to those who are ill. When we see others who seem under the weather, we experience a powerful emotional response—disgust—and do our best to avoid those who might be contagious. Now a study shows that seeing sick people can even prompt changes in the immune system.

Researchers at the University of British Columbia showed subjects one of two different slide shows—either a depiction of people brandishing guns or images of individuals who were obviously ailing. Immediately after the subjects viewed the slide shows, researchers drew their blood, exposed each sample to bacteria and then measured the levels of a substance known as interleukin-6 (IL-6), which is secreted by white blood cells as a response to stress or trauma. Although the subjects rated the gun photographs as being more stressful than the illness images, the blood work told a different story. Whereas the gun images prompted a mere 7 percent increase in IL-6, levels of the substance were elevated 24 percent after viewing pictures of sick people.

“It makes evolutionary sense that the immune system would respond aggressively only when it’s really needed,” says Mark Schaller, a psychologist and co-author of the study. “If I see a bunch of sick people, maybe a big infection is around, and I better kick my immune system into high gear.” It is unclear exactly how an image gets trans­lated into a mustering of immune cells, Schaller says, but many neurochemicals connect the brain to the immune system—more studies are needed to tease out the exact chain of events.