Exacerbations are a common feature in moderate-to-severe chronic obstructive pulmonary disease (COPD). Morbidity, mortality and health-care costs of these patients largely result from exacerbations. The most common causes of an exacerbation are infection of tracheobronchial tree. Among them, Streptococcus pneumoniae is the most frequently isolated organism, accounting for 5-25% patients of COPD, while it is also the most commonly identified cause in community-acquired pneumonia (CAP), accounting for 16.5-38.9% of CAP patients.