Over the past decade, the so-called hygiene hypothesis has received much attention. According to this hypothesis, the development of asthma is due in part to a lack of exposure to infections and microbial products early in life. There is widespread evidence that the number of older siblings affects the risk of hay fever and eczema: the risk decreases as the number of older siblings increases. Whether this effect is attributable to an increase in the number of infections transmitted by unhygienic contact with older siblings remains a matter of debate. Other, unknown factors associated with an increased number of pregnancies may also have a role in the development of atopy, supporting the importance of prenatal exposure for the inception of this condition. The role of viral infections early in life in the development of asthma has been fiercely debated. Viruses are potent triggers of exacerbations of asthma, and the inability to restrict the symptoms of rhinovirusinfections to the upper respiratory tract may be considered a hallmark of asthma at all ages. Moreover, infection with respiratory syncytialvirus can result inbronchiolitis in infancy, which, if severe, further increases the risk of subsequent wheezing episodes up to school age. Host factors such as reduced lung function and immature immune responses at birth may contribute to the expression of asthma induced by viral infections. Population-based studies assessing infectious exposure either indirectly, by recording whether a child has attended day care, or directly, by performing serologictests for viruses (hepatitis A virus and herpes simplex virus) and other microorganisms (Helicobacterpylori and Toxoplasmagondii), have found that exposure to infectious agents protects against asthma. Chronic infestation with helminths may also confer protection, but short-lived episodes of infestation may exacerbate atopic disorders.88 Much of the inverse association between infections and asthma may be attributable to atopy.