Asthma is a disease of hyper-reactive or twitchy airways. Asthmatic patients develop narrowing of their airways or bronchospasm following exposure to a variety of allergic irritants or other stimuli. Viral infections are one of the most common agents which increase bronchial hyper-reactivity and result in wheezing. They do so through a variety of mechanisms including perhaps allergy based effects as well as direct damage to the lining of the lungs which can then lead to bronchospasms. Viral infections are important triggers for asthma, particularly in children.

It is estimated that nearly half of the wheezing episodes in children 5 years of age or less are related to viral infections, especially those with RSV, the respiratory syncytial virus. For reasons that are not well understood, adults are less likely to develop wheezing during viral infections than children. It is important to consider bronchospasm as an important result of viral respiratory infection, as optimal therapy for such episodes might include inhaled bronchodilator medications.