Response to infections in patients with asthma and atopic disease: an epiphenomenon or reflection of host susceptibility?

Associations between respiratory tract infections and asthma inception and exacerbations are well established. Infant respiratory syncytial virus and rhinovirus infections are known to be associated with an increased risk of asthma development, and among children with prevalent asthma, 85% of asthma exacerbations are associated with viral infections. However, the exact nature of this relationship remains unclear. Is the increase in severity of infections an epiphenomenon, meaning respiratory tract infections just appear to be more severe in patients with underlying respiratory disease, or instead a reflection of altered host susceptibility among persons with asthma and atopic disease? The main focus of this review is to summarize the available levels of evidence supporting or refuting the notion that patients with asthma or atopic disease have an altered susceptibility to selected pathogens, as well as discussing the biological mechanism or mechanisms that might explain such associations. Finally, we will outline areas in need of further research because understanding the relationships between infections and asthma has important implications for asthma prevention and treatment, including potential new pathways that might target the host immune response to select pathogens.