Figure.

Contributions of viruses and bacteria to the development of asthma

Four essential host factors (purple) are formed both prenatally and postnatally depending on genetic background and environmental exposures. Although viruses and bacteria primarily interact with the mucosa, there is also interaction with the systemic immune response, local changes of which will be further boosted by atopic sensitisation, allergen exposure, and continuous infection. Transient wheeze is triggered by virus infections (red bolt) on the basis of altered airway function, but will be outgrown by children. Development of asthma might be determined by one or a combination of the four primary host factors and prenatal and postnatal environmental exposures, which might contribute to (red) or protect from (green) the development of asthma. HRVs=human rhinoviruses. RSV=respiratory syncytial virus. ETS=environmental tobacco smoke.