showed that children's who had at least two mild virus infection before their first birthday were only half as likely to be diagnosed with asthma or related allergies when they were seven,
compared with children who had been ill only once or not at all.
This finding supports he hypothesis that today's ultra-clean lifestyle is the reason for an increase in allergies in the general population in developed countries.
It is possible, although this is not yet established, that contact with the bacteria in soil and water is needed for cells in human immune systems to develop correctly. So parents should not overreact if their children sometime get muddy or dirty when playing outside.