Present knowledge suggests that asthma is a heterogeneous condition. Different genetic backgrounds may or may not express themselves as asthma-like symptoms, depending on both the nature and the timing of exposures in the individual. Although particular attention has been paid to environmental factors that may increase the risk of asthma, it is equally important to understand 'protective' exposures that may have decreased during the last decades and may thus explain the reported increases in asthma prevalence. Among these 'protective' factors are certain infections and the role they may have in the inception of allergics and asthma, particularly when they occur at critical times in the development of the immune system.