http://infectiousdiseases.about.com/b/2009/04/30/swine-flu-death-by-influenza.htm
“In severe cases of untreated influenza, the infection can lead to pneumonia, which is the main cause of influenza death. In some cases of influenza, damage to the respiratory tissue allows other infectious microbes to invade and cause secondary infections, leading to bacterial pneumonia, which can also be fatal when untreated. There are an estimated 36,000 deaths due to seasonal influenza each year in the US.”
I misspoke; secondary bacterial pneumonia is the MAIN cause of death from influenza; I omitted Cytokine Storm, which affects young adults.
actually if you read your own source, influenza pneumonia is the main cause of death. (viral pneumonia) It can not only kill directly but also leads to ARDS, (adult respiratory distress syndrome) which can be very nasty.