He cites the hygiene hypothesis as one that is widely supported by the medical profession.

BBC: NEWS | Health | Rise in multiple allergy patients

One of the more popular theories is "the hygiene hypothesis, " which says that exposure to germs and parasites in early childhood somehow prevents the body from developing certain allergies.

NPR: Study: Food, Skin Allergies Increasing In Children

There is a theory, called the "hygiene hypothesis", backed by Dr Djukanovic, which suggests measures such as the overzealous cleaning of houses has meant immune systems lack practice fighting bacteria and viruses.

BBC: Asthma vaccine hope

The most likely explanation could involve the "hygiene hypothesis, " which suggests that improved sanitation methods and efforts to keep germs at bay may deprive young, still-developing immune systems from being trained to recognize and react appropriately to the right types of irritants.

CNN: American kids have higher allergy risk