The common cold is the world's most widespread viral infection.

The results could offer some consolation to parents whose children regularly bring home colds from their nursery.

It also backs a long-held theory among some paediatricians about children's immunity.

The common cold is the most widespread viral infection in the world, with each person suffering between two and five colds each year.

Turnaround

With more than 200 known different viruses able to cause a cold, the UK population as a whole can expect to suffer 120 million colds each year.

The US study looked at 991 children.

They found children in nursery day care suffered almost double the amount of colds at the age of two, as those cared for at home.

A body's immune system is working fairly effectively early on so a long-lasting immunity to colds can be built up

Dr Nigel Hickson

But there was a turnaround from the ages of six to 11, when those who had attended nurseries had about one third as many colds as the other home-cared group.

The differences seemed to disappear by age 13, when both groups had the same amount of protection against colds.

Lead author of the study Dr Thomas M Ball, associate professor of paediatrics at the University of Arizona College of Medicine, said: "Personally, I've been telling people this for years because there is so much angst on the part of parents."

He said the findings should reassure parents who feel guilty about putting their children into nursery day care.

'Silver lining'

He said: "They should know the child's immune system is learning from this experience, and that (it) will come back to protect them later."

In an editorial accompanying the study, Dr Abraham Bergman of Harborview Medical Centre and the University of Washington, said the study proved there was a "silver lining" to having colds early on.

The benefit to colds in the toddler years is that kids miss less school later, when it counts

Dr Abraham Bergman

He said: "The benefit to colds in the toddler years is that kids miss less school later, when it counts."

Dr Nigel Hickson, chairman of the Primary Care Virology Group in the UK, said the research results "made sense".

He said: "There are so many snotty noses going around at nursery that children get exposed to different bugs early on.

"A body's immune system is working fairly effectively early on so a long-lasting immunity to colds can be built up."

'Nasty bugs'

But he stressed that with so many viruses causing colds, children can still pick up germs to which they are not immune.

And he warned that more "nasty bugs" such as measles would have a different effect.

He said: "If children - particularly those under one - get exposed to these kind of bugs they can overrun the body very quickly, before it has a chance to defend itself."

He said the danger of exposure to unfamiliar bugs often goes hand in hand with moving to a new social environment.

"It is the reason why we vaccinate undergraduates against Meningitis C when they start college because they come into contact with so many new viruses," he said.

The American study is published in February's Archives of Paediatrics and Adolescent Medicine.