New research shows that even just seeing a person who is sick "triggers" a biological response in our bodies to start preparing for illness.
A study by Patricia Lopes, an assistant professor of biology at Chapman University in California, revealed that bodies anticipate the possibility of infection when seeing another person's symptoms.
In the publication Functional Ecology, Lopes said it appears the human immune systems started preparing for illness early.
"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 behaviour of many others," she said in a press release.
The research on people's abilities to sense illness was conducted following a separate study by Lopes that was published in Trends in Ecology & Evolution.
In this study, Lopes suggested that when healthy animals interacted with others who showed signs of sickness, their bodies "activated" their immune responses.
"Sickness cues such as behaviours and odours are also directly altered by the inflammatory response and its effects on neurophysiology and metabolism," the study on animals reads.
More research is needed to determine how long humans' anticipation responses last, and whether they help build a barrier to avoid illness, researchers said.
Lopes said that while being exposed to people who are sick and having the response triggered may act as some sort of defence mechanism, it does not replace preventative measures like vaccines.
"This phenomenon raises questions about the interconnectedness of individuals within a social group and how the perception of sickness can influence the health and behaviour of others," the press release reads.