Staying optimistic 'keeps you healthy'

Staying optimistic keeps you healthy, according to a new study which also found that a "glass-half-empty" attitude can damage your immune system.

The research showed that when you are feeling more pessimistic you will be less able to fight off viruses.

It is thought that everything from marital spats to job stress can make us more likely to fall ill.

Scientists tracked the outlook of students over the course of a year and tested their immune system during that time.

Professor Suzanne Segerstrom, who led the research, said: "When people were feeling more optimistic, they also had rather stronger responses to an immune challenge.

"To show that a single person - with the same personality and genes - has different immune function when they feel more or less optimistic provides a stronger link between the two than comparing an optimistic with a pessimist.

"I think that when people are feeling optimistic about a specific, important challenge they are facing, they may be less vulnerable to certain health problems, like viral infections.

"When they are feeling pessimistic they are more vulnerable.

"Why this happens is still a bit of a mystery.

"When people felt more optimistic, they also felt more positive emotions, and this certainly accounted for part of the effect."

However, Prof Segerstrom said the neurobiological effect of positive emotions was harder to track and measure than negative emotions.

Her team recruited 124 first year law students and got them to complete five questionnaires and immunity checks over the course of a year.

The questionnaires measured students' optimism by asking how closely they identified with statements such as "I will be less successful than most of my classmates."

To test immunity, the students had a dose of dead mumps virus or candida yeast injected under the skin of the forearm.

These harmless cocktails trigger a cellular immune response, resulting in a small bump at the injection site.

By measuring the bump, researchers can estimate the strength of the immune response.

As the students experienced classes and exams their optimism levels rose and fell and so did their immunity, or .

When optimism went up, so did their immune response. When pessimism crept in, the immune system weakened.

To illustrate the strength of the link, Prof Segerstrom said it was greater than the effect calcium has on bone mass, or blood pressure medicine has on the risk of stroke.

Prof Segerstrom, a psychologist at the University of Kentucky, said the positive emotions caused by optimism were likely to improve the immune system.

The research is published in the journal Psychological Science.