Up to 10% of adults and 35% of children worldwide suffer from asthma. The incidence of asthma is also higher in children than in adults, possibly owing to a "cohort" effect. Recent population-based studies show that the increase in the prevalence of asthma noted worldwide in the past 30 years has started to subside in industrialised countries. This phenomenon has been accompanied by a parallel increase in the use of asthma medications. Possible explanations for this increase include the "hygiene hypothesis", which implicates diminished exposure to microbial pathogens but was recently challenged by new biological and immunological findings. Asthma might result from the interactions between individual and environmental factors (diet, air pollution, etc.), but further investigations are needed to understand the precise pathophysiological mechanisms of asthma.