Viral–bacterial respiratory coinfections have been reported for more than a century, including during the 1918 influenza pandemic. However, only recently have we learned through advances in microbiologic diagnostics that the actual rate of community-acquired pneumonia (CAP) coinfections is much higher than previously reported, reaching a prevalence of 20%–30% in more contemporaneous cohorts. CAP coinfection is more complex than it has been historically known, including distinct immune-modulating effects by different viral strains, possible dual active infections, interactions of the viral pathogen with the host pulmonary microbiome, and variations in clinical presentation and prognosis that may be secondary to direct pathogen effect versus host immune response versus pathogen–host interactions.

Approximately 30%–40% of patients hospitalized with laboratory-confirmed influenza are diagnosed with acute lower respiratory infection. Patients who develop pneumonia are more likely to be young (aged <5 years), old (aged >65 years), have chronic lung or heart disease and a history of smoking, and are more commonly immunocompromised. However, compared with seasonal epidemics, influenza pandemics are associated with a higher rate of hospitalization due to respiratory failure and intensive care unit (ICU) admissions in healthy and young adults.