That's the word from a research team led by Richard Davidson of the University of Wisconsin-Madison. Their findings are being published this week in the online edition of Proceedings of the National Academy of Sciences.
The scientists were studying links between a person's psychological state and their immune response.
To do this, they asked 52 volunteers, male and female, to write about the worst time of their lives and the best time. Researchers then measured their brain activity electronically.
Increased activity in the right prefrontal area of the brain has been associated with stronger emotional response and depression.
After the testing the individuals were given flu shots and were tested two weeks, four weeks and six months later to determine their reaction to the shots.
Individuals who had shown greater activity in the right prefrontal part of the brain later had lower amounts of flu antibodies in their blood, indicating a weaker response by their immune system.