Asthma has become a very common disease and despite new treatments continues to cause very large numbers of hospital admissions, school absences and so on. Recent evidence from many parts of the world has established that the single most important risk factor for perennial asthma is sensitization to one of the major indoor allergens: dust mite, cat, dog, or cockroach. However, there is also striking evidence about the role of rhinoviruses and ozone as enhancers of the immune response. Over the past 2 years, the ways in which these enhancers interact with the primary immune response both to cause asthma and to influence the severity of the disease have started to become clear.