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Respiratory illnesses account for nearly two thirds of the total illness in a community,¹ account for as much as 40% of the problems seen in a pediatric practice,^2,3 and are responsible for over one third of school absences.⁴ Despite major advances in the microbiology of respiratory disease, why and how a child becomes ill remain poorly understood. In over half of respiratory illnesses, complete cultures fail to yield an etiologic agent.⁵ Conversely, 30% of a school-age population can harbor group A streptococci without developing symptoms,⁶ three quarters of preschool children infected with Mycoplasma pneumoniae remain asymptomatic,⁷ and as many as 42% of upper respiratory tract cultures from well children yield pneumococci.⁸ Furthermore, there is no satisfactory explanation for the observation that certain children are predisposed to more frequent or more severe respiratory illnesses.⁹

The limited ability of …