Last updated at 21:46 11 June 2007

Giving babies antibiotics doubles the chances they will suffer from asthma by their seventh birthday, research has revealed.

A study of more than 13,000 children added to the growing evidence that exposure to germs in the first few months of life aids the development of a healthy immune system and protects against allergies.

Researchers also found that infants exposed to the hairs of a family dog before their first birthday were less likely to suffer from asthma later in life.

Many doctors believe that the modern obsession with cleanliness could be linked the dramatic rise in allergies over the last few decades.

The study, published in the journal Chest, looked at the medical histories of 13,116 children from birth to age seven.

Two-thirds were given an antibiotic before their first birthday.

Researchers found they were far more likely to have asthma than those not given the drugs --and the more courses of antibiotics they were given, the greater the chances.

When the scientists excluded those given antibiotics for breathing problems - who might be expected to have a higher chance of developing asthma --the connection appeared even stronger.

Asthma in seven-year-olds was nearly twice as likely in children given an antibiotic for a nonrespiratory tract infection than in children who had been given no antibiotics at all.