“I started to feel sick just looking at them!” Anyone saying this after spending time in proximity to a person who appeared ill may assume their reaction is purely psychological. But research now suggests that the human body may initiate a biologic immune response upon encountering a sick person.

Previous research in animals showed that interaction with sick animals led to activation of molecular pathways of immune response and changes in egg composition in healthy animals. Now, researchers have demonstrated similar biologic responses in healthy humans who observe a sick individual. Findings appear in the journal Trends in Ecology & Evolution.

“This research has helped unveil another level of the hidden ripple effects of infections, showing that when one individual falls ill, it’s not just their problem – it’s a complex story that can impact the health and behavior of many others,” says Patricia Lopes, an assistant professor of biology at Chapman University, and author of the current study.