Here's a new report that suggets that children who are exposed to bacteria transmitted by dogs may be protected children from childhood asthma, because they bring about changes in our immune system which protect from a virus or because they change our gut flora. They carried out experiments on mice:

The researchers fed mice house dust from homes with dogs. They then exposed these mice to RSV and found that the mice did not show symptoms of infection -- airway inflammation and increased mucus production. They also compared the intestinal bacteria of dust-fed mice to mice that hadn't been fed dust and found differences in the types of bacteria living in the GI tract. These differences were seen whether or not the non-dust fed mice had been exposed to RSV.