Causes

• Influenza A virus
• Haemophilus influenzae
• Chlamydia psittaci
• Legionella pneumophila
• Coxiella burnetii  
• Staphylococcus aureus 

Note:

Other causes are less common

infecting organismc	% (95% CI)
Influenza A virus	5.4% (3.3% to 7.6%)
Haemophilus influenzae	4.1% (2.2% to 5.9%)
Chlamydia psittaci	2.3% (0.9% to 3.7%)
Legionella pneumophila	1.6% (0.4% to 2.8%)
Coxiella burnetti	1.1% (0.1% to 2.1%)
Staphylococcus aureus	0.9% (0.02% to 1.8%)
other known causes	3.6% (1.9% to 5.4%)
unknown microbiology	34% (29% to 38%)