Worldwide, Streptococcus pneumoniae is by far the most important pathogen for CAP. Other frequently isolated bacteria are Haemophilus influenzae and Staphylococcus aureus.11--3 Incidences of atypical pathogens, such as Mycoplasma pneumoniae, Chlamydia pneumoniae, and Legionella pneumophila, are generally lower than those of the afore-mentioned bacteria, although variations may be large.17, 18, 20, 23 Pseudomonas aeruginosa can be relevant in patients with structural lung damage, such as those with bronchiectasis or COPD.24 Most frequent viral causes of CAP include influenza virus and parainfluenza virus.23, 25 Viral pneumonias due to infection with influenza, respiratory syncytial virus, coronaviruses, parainfluenza virus, and even rhinoviruses can be life threatening in elderly and immunocompromised patients. Influenza pneumonia may be complicated by secondary bacterial infections caused by S. aureus, S. pneumoniae, H. influenzae, or other Gram- negative pathogens.21, 26

Recently, coronaviruses have been recognized as causes of severe lower respiratory tract infections. The SARS coronavirus caused severe CAP associated with high mortality rates, even in previously healthy adults.27, 28 Non-SARS coronaviruses such as the coronavirus OC43 have been associated with lower respiratory tract infections in children and adults.25, 29-31 Human metapneu-movirus is increasingly recognized as a cause of respiratory failure in children and of pneumonia in the elderly.32, 33 Importantly, up to 60% of episodes of CAP remain of unknown etiology.6

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