A study at Imperial College London in mice suggests that picking up a particular type of virus during this period makes re-infection far more serious.

However, many doctors will disagree with the advice to "wrap babies in cotton wool" and try to prevent any infections early in life.

A paediatric expert has told BBC News Online that parents should be "cautious" about applying this advice.

Putting aside the extreme difficulty in keeping a baby from picking up infections, there is some evidence that exposure to certain infections may actually be good for children.

It has been suggested that extensive contact with other children, and by implication an increase in exposure to common infections, may protect from a variety of illnesses - some serious.

Challenging the developing immune system may make it stronger, say some experts.

Severe infection

The virus examined in the Imperial College study is Respiratory Syncytial Virus (RSV), which causes a common lung infection.

In some children, RSV causes a severe condition called bronchiolitis, which increases the chance of developing asthma.

However, scientists do not fully understand why most children get over RSV infections with no problems, but a small number develop bronchiolitis.

The researchers believe that children who have been exposed early in life to RSV can develop a worse infection if they encounter the virus again.

They experimented with mice, infecting them with RSV either at one day old, or at four or eight weeks.

They were then reinfected at 12 weeks old.

Those given "priming" just after birth had a far more severe reaction to the later re-infection.

They lost more weight and their lungs showed signs of being more inflamed - as a direct result of the response of their immune systems.

Delaying tactics

Professor Peter Openshaw, who led the research group, suggested that the same thing could happen in humans.

I would be very cautious about extrapolating these studies in mice directly into humans

Professor Rosalind Smyth, University of Liverpool

He said: "Although there is still no way to prevent babies being infected by RSV, keeping people with colds away from young babies could reduce the chances of infection.

"Merely delaying infection beyond the first six months could have a significant impact on the later health of a child."

However, it is still not clear whether developing bronchiolitis makes children more susceptible to asthma - or whether children more naturally susceptible to asthma are also more prone to getting bronchiolitis when they catch RSV.

In the the Journal of Experimental Medicine, the researchers wrote: "Bronchiolitis may be the first manifestation of a predisposition to recurrent respiratory disorders - or severe RSV disease may lead directly to chronic, persistent or delayed disease."

Professor Rosalind Smyth, a consultant in paediatric medicine at the University of Liverpool, urged parents to be cautious about the findings.

She told BBC News Online: "These are very interesting findings - and could certainly be important.

"However, I would be very cautious about extrapolating these studies in mice directly into humans.

"In addition, it is very difficult for any mother to stop their children getting colds - particularly if there are siblings."