The father of a 3-year-old boy, who has just started preschool, brings the boy to your office for “his fourth cold in a row.” The boy was back to his baseline last week but now has congestion and rhinorrhea. On physical examination, he is afebrile and has no signs of otitis media or lower respiratory tract involvement. His father expresses frustration with these frequent illnesses and asks if there is something else he can do. As a general pediatrician, you often are confronted with this clinical scenario. You recall a recent study of probiotics for the prevention of colds and influenza-like illnesses and review it more carefully.

Leyer and associates (1) conducted a prospective, randomized, double-blind, placebo-controlled study of 326 children, in which 3- to 5-year-old children received one of three products for 6 months. Probiotic I consisted of one strain of probiotic, probiotic II consisted of two...

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