Two types of pneumonia are well recognized during influenza: primary viral pneumonia and secondary bacterial pneumonia. Primary viral pneumonia occurs after a typical onset of influenza with rapid progression of dyspnea and cough leading to acute respiratory distress syndrome. Treatment consists of respiratory assistance, but mortality is high. Secondary bacterial pneumonia occurs more frequently in the elderly and in patients with chronic pulmonary diseases. Staphylococcus aureus, Streptococcus pneumoniae and Haemophilus influenzae are the most frequently isolated bacteria. After an initial phase of clinical improvement, manifestations of bacterial infection with pulmonary consolidation occur. The outcome is favorable with antibiotics but depends on the patient's underlying conditions.