A study that saw mice contracting flu to prevent asthma symptoms has supported an idea that children might be kept too clean in the developed world.
Scientists observed that mice who contracted the influenza A virus were protected against asthma as adults, while adults catching flu were still susceptible to the condition.
The reason for this change was traced to a sub-group of immune system cells called natural killer T-cells (NKT cells).
Some of these cells can maintain order in the immune system and prevent extreme reactions that cause allergies like asthma. In baby mice with flu, but not adults, there were more of these cells.
The results, which have been published in the Journal of Clinical Investigation, support the "hygiene hypothesis" of many US and Japanese scientists.
They believe that the rise of asthma in recent years could be because of reduced exposure to bacteria or illnesses as a child.
It is thought that early infections can "prime" the immune system to prevent it reacting to allergy triggers in later life, according to the theory.
The research also showed that NKT cell protection against asthma could be induced by exposing baby suckling mice to a molecule from the stomach bug Helicobacter pylori.
The scientists, led by Dr Dale Umetsu, from Harvard Medical School in Boston, US, wrote: "Our results suggest that infection with certain micro-organisms can prevent the subsequent development of asthma and allergy by expanding the relative proportion of a specific subset of NKT cells, thus providing an immunological mechanism for the hygiene hypothesis."
"Sorry, I need more research to be convinced with this theory. I disagree with this because of personal experience of having 1 child who has asthma and not the other two. No change in life style condition for the 3 children. The only other factor that differs amongst the sibling is the child who has a tendency to have asthma was a premature baby and investigations(blood test) showed that he has severe allergy to the egg yolk, so a protein allergy that causes his asthma. He still gets bouts of this when food allergy awareness is neglected" - Anne Majumdar, Wilts
"I think asthma is sometimes a familial trait and therefore this would not be the case for those whose families have several generations affected by asthma" - Frances Harris, Haywards Heath