Complications of seasonal and pandemic influenza.

Influenza is a seasonal viral infection associated with significant morbidity and mortality. In 2009, a novel H1N1 influenza A virus emerged and has been classified as a pandemic. In contrast to seasonal influenza, severe disease from pandemic H1N1 seems concentrated in older children and young adults, with almost no cases reported in patients older than 60 yrs. Although patients with underlying cardiopulmonary disease remain at risk, 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. Treatment of pneumonia should include empirical coverage for this pathogen. Neuromuscular and cardiac complications are unusual but may occur.