Co-infection in severe influenza: a new epidemiology?

In 2009, a novel influenza A (H1N1) virus emerged in Mexico and caused human infection, including severe pneumonia in young and previously healthy adults [1]. Since 2009, the virus has continued to circulate, causing cases of viral pneumonia and acute respiratory distress syndrome requiring intensive care unit (ICU) admission. Other serotypes [influenza B, A (H3N2)] circulate concomitantly and are also responsible for cases of severe acute illness requiring ICU admission [2]. Although primary viral pneumonia may evolve towards acute respiratory distress syndrome and death, bacterial co-infection is frequently described in these cases, may contribute to the development of ARDS and respiratory failure, and is clearly associated with higher mortality.