No reason to feel optimistic

Conversely, those who are depressed, nervous or angry are more likely to complain about cold symptoms - whether or not they get bitten by the cold bug.

A team from Carnegie Mellon University, Pittsburgh, found people who had a positive emotional attitude were not infected as often and experienced fewer symptoms than people with a negative emotional style.

There are a lot of interactions going on in the body which we just don't fully understand

Dr Nigel Higson

After their assessment, each volunteer got a squirt in the nose of a rhinovirus - the germ that causes colds.

The researchers kept the subjects under observation for five days to see whether or not they became infected and how they manifested symptoms.

Tests showed that positive people were no less likely to be infected with the virus.

However, infection seemed to produce fewer signs and symptoms of illness.

Lead researcher Dr Sheldon Cohen said: "We found that experiencing positive emotions was associated with greater resistance to developing a common cold.

"But a negative emotional style had no effect on whether or not people got sick."

Immune system

Dr Cohen believes the findings suggest that a positive outlook may impact on how effective the immune system is at fighting disease.

He said that a more upbeat attitude may also help to reduce the risk of other infectious diseases.

Dr Cohen told BBC News Online: "The symptoms of a cold are caused by the release of chemicals such as cytokines, histamines and bradykinins.

"The release of these chemicals is to some extent under the control of hormones that are produced when we experience various emotions.

"We think that the levels of these hormones in happy people may partly protect them from developing symptoms of cold when infected by a cold virus."

Dr Nigel Higson, a GP and chairman of the Primary Care Virology Group, said there was a lot of anecdotal evidence to suggest that stress had an impact on the immune system.

He said: "It seems that the immune system is affected by your wellbeing, and if you are free from stress it is able to fight disease more rapidly."

Dr Higson said it was possible that chemicals in the brain may have a direct impact on the functioning of immune system chemicals.

However, he said: "There are a lot of interactions going on in the body which we just don't fully understand."

The research is published in the journal Psychosomatic Medicine.