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