The idea is that your mood can actually effect your body's physical health because there is a direct link between being happy and being healthy. Apparently your immune system responds more favorably when you're in a positive state.
Yahoo reports that researchers came to this conclusion by observing 138 participants over a six-week period. Patients were questioned about their moods, physical activity, diet, and sleep patterns to find a direct link between psychological and physical behaviors and their body's response to the vaccine. Researchers also took into account how many flu antibodies each person developed at four and 16 weeks after receiving the shot. The results showed that those leading a happier, more positive lifestyle, and who had a more positive outlook on the day of vaccination, had higher levels of influenza antibodies. [Elite Daily]