Asthma is a chronic inflammatory disorder of the airways. Allergen-induced airway inflammation plays a central role in the pathogenesis of the disease whereas infections are known to be important triggers of acute exacerbations. Chronic or latent infections may also influence and enhance the local inflammation. The aim of this review is to give an overview of the different pathogens and to present the current understanding of the role of respiratory viruses and atypical bacteria in the pathogenesis of the disease.