The most severe form of respiratory complication in influenza virus infection is:

Primary influenza viral pneumonia is the least common but most severe of the pneumonic complications. The most common bacterial pathogens causing secondary bacterial pneumonia are Streptococcus pneumoniae, Staphylococcus aureus, and Haemophilus influenzae. Ref:Harrisons Principles of Internal Medicine, 18th Edition, Page 1496