Respiratory Virus Infection of the Lower Airways and the Induction of Acute Asthma Exacerbations
Respiratory viral infections are among the most common causes of physician consultation, especially among the pediatric population. While in most occasions the upper airway is affected (i.e., a common cold), it is frequently difﬁcult to deﬁne clinical and/or pathological limits between upper and lower airway involvement. Hence, in the context of viral infections, it is not overstated that the human airways could be viewed as a continuous system starting from the nostrils and ending at the alveoli (1). Although signiﬁcant differences between anatomical sites could be pointed out, viral agents with a tropism for the respiratory system infect and/or can produce symptoms in both upper and lower airways. In addition to its importance in purely infectious disease states, this observation is relevant to the sequence of events leading from a common cold to the exacerbation of reactive airway disease, mainly asthma. During the last decades, a remarkable increase in the prevalence of asthma in afﬂuent societies has been documented. Asthma now affects over 150 million people worldwide and costs more than tuberculosis and acquired immunodeﬁciency syndrome (AIDS) combined (2). Although there is no doubt that a multitude of genetic and environmental factors are involved in the induction and progression of asthma, upper respiratory viral infections (URIs) are the most common trigger
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of acute exacerbations. This correlation has long been suspected, as it is a daily observation of practicing physicians.