We are slowly beginning to unravel the complex interactions between mental and physical health. Researchers have found a wealth of evidence that positive emotions can enhance the immune system, while negative emotions can suppress it. For example, individuals can take up to a year to recover a healthy immune system following the death of their spouse, and long-term caregivers have suppressed immune systems compared with persons in the general population.

Studies on survivors of sexual abuse and those with post-traumatic stress disorder suggest they have elevated levels of stress hormones, as do students at exam time. In these groups of people and others experiencing loneliness, anger, trauma and relationship problems, infections last longer and wounds take longer to heal. However, having fun with friends and family seems to have the opposite effect on our immune systems. Social contact and laughter have a measurable effect for several hours. Relaxation through massage or listening to music also reduces stress hormones.