 Most complications have occurred among previously healthy individuals , with obesity and respiratory disease as the strongest risk factors . Pulmonary complications are common . Primary influenza pneumonia occurs most commonly in adults and may progress rapidly to acute lung injury requiring mechanical ventilation . Secondary bacterial infection is more common in children . Staphylococcus aureus , including methicillin @-@ resistant strains , is an important cause of secondary bacterial pneumonia with a high mortality rate ; streptococcus pneumoniae is the second most important cause of secondary bacterial pneumonia for children and primary for adults . Neuromuscular and cardiac complications are unusual but may occur . 
