Medical and scientific experts now agree that bacteria, not influenza viruses, were the greatest cause of death during the 1918 flu pandemic.
…That pneumonia causes most deaths in an influenza outbreak is well known. Late 19th century physicians recognised pneumonia as the cause of death of most flu victims. While doctors limited fatalities in other 20th-century outbreaks with antibiotics such as penicillin, which was discovered in 1928, but did not see use in patients until 1942.
…McCullers’ research suggests that influenza kills cells in the respiratory tract, providing food and a home for invading bacteria. On top of this, an overstressed immune system makes it easier for the bacteria to get a foothold.
https://www.newscientist.com/article/dn14458-bacteria-were-the-real-killers-in-1918-flu-pandemic/