Neonates: Group B streptococci, E. coliChildren: Mycoplasma, C trachomatis/pneumoniae, S pneumoniaeAdults: S pneumoniae, H influnzae, anaerobes, viruses (eg, influenza)Elderly: S pneumoniae, influenza, anaerobes

Alcoholics: Klebsiella, anaerobes usually due to aspirationAspiration: AnaerobesDrug users: S pneumoniae, S aureusImmunocompromised: S aureus, gram - rods, fungi, P jirovecii (with HIV)Cystic fibrosis: PseudomonasNosocomial: S aureus, PseudomonasPostviral: S pneumoniae, S aureus, H influenzae

Atypical: Mycoplasma, Chlamydia, Legionella