National Center for Biotechnology Information NCBI NLM MEDLINE diagnosis humans pubmed abstract nih national institutes of health national library of medicine adult female male age factors aged middle aged young adult Prospective Studies 80 and over severity of illness index community-acquired infections / microbiology pneumonia differential streptococcus pneumoniae / isolation & purification* multivariate analysis viral / microbiology netherlands bacterial / diagnosis* bacterial / microbiology community-acquired infections / diagnosis* viral / diagnosis* reverse transcriptase polymerase chain reaction pmid:24344207 doi:10.1099/jmm.0.067108-0 elisabeth g w huijskens marion koopmans john w a rossen coinfection influenza a virus / isolation & purification*