The parainfluenza viruses are important because of the serious diseases they can cause in infants and young children. Parainfluenza 1 and 3 are particularly common in this regard. Overall, the group is thought to be responsible for 15 to 20% of all nonbacterial respira- tory diseases requiring hospitalization in infancy and childhood. Immunity to reinfection is transient; although repeated infections can occur in older children and adults, they are usually milder than the illnesses of infancy and early childhood.
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