The rational goes as follows: the rural & farming environment exposes children to a far wider range of microbes compared with children in a city environment so we should be able to detect differences between the two population with regard to their health - in this case asthma and atopy. If these microbes are bad then those children should suffer from more asthma. The study showed the opposite to be true - children exposed to a wider range of microbes - including moulds such as Aspergillus - suffered from less asthma.
There is similarity to the report published 2 years ago that children who played in dirt more often were prone to less allergy (BBC report). In that study researchers found that Staphylococci microbes actually protected children from over-reacting to common allergens.
In this more recent study the same has been found for Aspergillus and bacteria such as Listeria. Exposure at a young age to these microbes seems to protect children against developing asthma.
Perhaps that camping holiday in the countryside does more good for your children than simply providing fresh air and exercise, perhaps it exposes them (especially at a young pre school age) to beneficial microbes which help prevent asthma.
Does this mean that living in a damp house is beneficial to your child? Absolutely not. It has been demonstrated several times that damp housing is bad for the health of your child. There is no mention of the housing standards of the children involved in this study and perhaps children from farms had less damp houses? Regardless there is something about damp housing that is bad for our health.
Perhaps when this research proceeds further we will be able to identify particular microbes that are good, and some that are bad (as the preceding study on allergy did). Perhaps it will show that bad microbes tend to overgrow in damp housing.
At the moment all we have are conclusions based on identification of very broad groups of microbes potentially containing many hundreds of different microorganisms. We have a general observation that exposure to a broader range of microbes is good for atopy & asthma. We now need more specific work to be carried out. Nonetheless we have moved one step forward in the battle against asthma.