The prevalence and severity of allergic asthma is increasing worldwide. Epidemiological evidence suggests that the increase in disease prevalence is inversely associated with the occurrence of childhood infections. Specific pathogens, and pathogen-derived components have now been identified that can suppress allergic disease in mice and humans. These discoveries highlight the potential of these microorganisms as a new source of immunotherapeutics. The utility and mechanism of action of these pathogens or pathogen-derived components will be discussed in the context of providing a novel approach for the treatment of allergic airway disease.