Influenza virus infections are associated with considerable morbidity and mortality worldwide. In the US alone, despite the availability of vaccines and antiviral drugs, influenza causes approximately 200,000 serious infections that require hospitalization, and 36,000 deaths each year.

In typical seasonal influenza, most deaths occur in the elderly or in people whose immune systems have been compromised by disease or medication. In contrast, influenza pandemics infect up to 50% of the population, and a high number of deaths occur in previously healthy young adults. A common complication of influenza is secondary bacterial infection, which greatly increases the frequency of complications such as pneumonia and meningitis, and contributes to a much poorer prognosis. The most common secondary infections are due to the bacteria Streptococcus pneumoniae, Haemophilus influenzae (H. influenzae) and Staphylococcus aureus. However, the mechanism by which influenza infection predisposes to secondary bacterial infections is poorly understood.