Age	Common causes		Less common causes
Birth to 20 days	Bacteria		Bacteria
		Escherichia coli		Anaerobic organisms
		Group B streptococci		Group D streptococci
		Listeria monocytogenes		Haemophilus influenzae
				Streptococcus pneumoniae
				Ureaplasma urealyticum
			Viruses
				Cytomegalovirus
				Herpes simplex virus
3 weeks to 3 months	Bacteria		Bacteria
		Chlamydia trachomatis		Bordetella pertussis
		S. pneumoniae		H. influenzae type B and nontypeable
	Viruses			Moraxella catarrhalis
		Adenovirus		Staphylococcus aureus
		Influenza virus		U. urealyticum
		Parainfluenza virus 1, 2, and 3	Virus
		Respiratory syncytial virus		Cytomegalovirus
4 months to 5 years	Bacteria		Bacteria
		Chlamydia pneumoniae		H. influenzae type B
		Mycoplasma pneumoniae		M. catarrhalis
		S. pneumoniae		Mycobacterium tuberculosis
	Viruses			Neisseria meningitis
		Adenovirus		S. aureus
		Influenza virus	Virus
		Parainfluenza virus		Varicella-zoster virus
		Rhinovirus
		Respiratory syncytial virus
5 years to adolescence	Bacteria			Bacteria
		C. pneumoniae		H. influenzae
		M. pneumoniae		Legionella species
		S. pneumoniae		M. tuberculosis
				S. aureus
			Viruses
				Adenovirus
				Epstein-Barr virus
				Influenza virus
				Parainfluenza virus
				Rhinovirus
				Respiratory syncytial virus
				Varicella-zoster virus