Etiology of childhood pneumonia: serologic results of a prospective, population-based study. - Heiskanen-Kosma,T.; Korppi,M.; Jokinen,C.; Kurki,S.; Heiskanen,L.; Juvonen,H.; Kallinen,S.; Sten,M.; Tarkiainen,A.; Rönnberg,P.R.; Kleemola,M.; Mäkelä,P.H.; et al. (Pediatr Infect Dis J, 1998)