Nowadays, people bathe and wash their clothes more frequently than in the past, and thanks to vacuum cleaners homes are less dusty, too. One result of all these changes is that in their early lives children are exposed to fewer allergensーsubstances that can cause allergiesーand this means that their bodies cannot build up natural immunity to them.
The trend towards smaller families also means that young children encounter fewer allergens in the home. In fact, it is known that children who have older brothers and sisters are more resistant to allergies. The same is true of children who share their home with a pet. Such children are much less likely to develop the very common allergy to cat or dog hair, for example.