Recently, several authors have documented that respiratory infections may cause wheezing and acute exacerbation of asthma in children. Respiratory syncytial virus infections have been recognized to produce the first episode of wheezing in children who go on to develop chronic asthma. Furthermore, repeated infections caused by other common childhood viral pathogens have been proposed to affect responses of the immune system in such a way as to prevent the onset of allergic diseases and possibly asthma. Recently, it became clear that also infections by intracellular pathogens, such as Chlamydia and Mycoplasma, may cause acute and chronic wheezing in some individuals. In this review we describe the immunologic and clinical implications of the association between respiratory infections and asthma.