Researchers at the University of British Columbia have found that people can boost their immune systems just by looking at sickness. They studied three groups of people to see what their immune systems would do when they see certain types of images. One group looked at images of furniture, one group looked at images of violence, and one group looked at images of illness. The furniture and violence groups had no metabolic response to their subjects, but the virulence viewers showed a 25 percent increase in their levels of the immunity factor Interleuken-6, which responds to infections and injuries. The researchers think the psychological triggering of the immune system may be "presumably adaptive in origin". If people can boost their immune systems by thinking about it, it could explain positive effects from placebos, meditation, prayer, tribal rituals, and faith healing.