Two new pieces of research suggest they may be. Children are less likely to become sensitive to allergens if they have a dog in the home, according to German scientists, while an American study found that regular social interaction reduced children's risk of leukaemia by about 30%.

These findings seem to chime with popular perceptions of "cotton wool kids" and the belief that our increasingly sterile, urban and risk-averse society paradoxically creates new dangers for our children.

The causes of leukaemia are complex, and most childhood leukaemias begin with a genetic mutation, so the US study is far from conclusive. But there's an established hypothesis that if an immune system is not challenged early (our immune responses develop from birth to about age 12), it can increase the risk of developing allergies and illnesses.

Ken Jones, professor of immunology and allergy at the Cardiff School of Health Sciences, explains that our immune response has taken millions of years to develop, so rapid changes - notably, the proliferation of allergies - cannot be genetic.

Immune systems have been transformed by modern life. Young children tend not to play outside or in extended families. This limits their exposure to bacteria, viruses and parasites, and prevents them developing an immune response.

Our homes have also changed: asthma may be triggered because central heating reduces air quality in our hermetically sealed homes, while soft furnishings have caused the proliferation of dust mites.

So get fresh air in your home and let your children play outside - and mix with other kids. "No one is suggesting you deliberately expose your children to disease-forming organisms," says Jones, "but being out in the dirt is probably not too bad a thing."