Stress is being increasingly blamed for society's ills, but short, sharp bursts can be good for your health, research suggests.

A short burst of stress, for example by having to give a speech in public, may actually strengthen the body's immune system. While long term stress, such as living with a permanent disability or chronic illness, can reduce the ability to fight infections, some psychological stress can have the opposite effect.

Dr Suzanne Segerstrom of the University of Kentucky and Dr Gregory Miller looked at 300 scientific papers covering nearly 19,000 people. They found that stressful situations which lasted only short periods appeared to tap into the primeval 'fight or flight' instincts of early man threatened by predators. That response boosted the body's front-line defence against infections from traumas like bites and scratches.

But long-term anxiety has the opposite effect. Situations that cause permanent stress, like caring for someone with dementia, appear to wear out the immune system leaving the person vulnerable to infection. The same effect could be caused by other damaging experiences, such as losing a spouse or suffering abuse as a child.

The key factor seemed to be knowing that the event causing the stress would end soon. Older people and those already suffering an illness were more likely to suffer damage to their immune systems.

Dr Miller says: 'Older people are definitely more vulnerable to stress. Either the mind or the body seems to lose some of its ability to fight back.

The issue now is whether the changes are severe enough and long enough to actually influence people's vulnerability to disease.'

Similar results have been achieved by British researchers. Dr Veronica Manyande, senior lecturer in psychology at Thames Valley University, tested patients facing surgery at St Mark's Hospital, Islington. One group was simply given information about their operations, the other was instructed in relaxation techniques before and after surgery.

All these patients were facing stressful medical procedures,' Dr Manyande says. 'Those in the relaxation group had much higher stress levels and they recovered more quickly.'

She concludes: 'Some levels of stress are good for you. You have to have some level of stress if you are a living human being. It really does strengthen your immune system which often doesn't get exercised enough.'

Her research shows that acute levels of stress are helpful, but chronic levels are likely to be harmful.

Philip Hodson, a fellow of the British Association for Counselling and Psychotherapy, agrees. He says it is well understood that healthy organisms respond well to relieved stress. But unrelieved stress can be bad for your health.

IIf you are forced to do an exam and you do well, or you go for a job interview and get the job, or perform in a play and the audience applauds - you feel great.'

But if you fail in any of those events, the results may be different. Depending on the individual and the personality, such defeats can be good because lessons are learned from them.