The 1918-1919 influenza pandemic was the most brutal flu outbreak in the 20th century and one of the deadliest pandemics in human history. There were an estimated 25 million deaths, though some researchers have projected that the death toll was actually between 40 and 50 million. Outbreaks occurred in practically every inhabited part of the world.
I do not think that another world-wide flu pandemic could occur in our current day. We now have access to vaccines and treatments that were not available in 1918. Flu shots are offered every year and more and more people are starting to get them. While this does not protect us from all strains of the flu, it does protect us from the most prevalent strains. People are also much more educated now on how disease spreads. They are also more educated on looking for signs and symptoms that are out of the ordinary and knowing when to go to the doctor. When this pandemic occurred, many people probably did not understand how contagious the flu is nor did they have access to the appropriate medical supplies to protect themselves.
I would not say that it is impossible for a world-wide flu pandemic to occur again. If the avian flu were to somehow make its way around the world, the results would probably be catastrophic. If the avian flu were to be weaponized and large populations were to be infected, the death toll would be insurmountable. But I believe that the flu being the next “Big One” is highly unlikely. I believe that the next widespread disease outbreak will be something like SARS. Maybe not SARS exactly, but something that people will get and start having symptoms, but not think much of and not take seriously until it is too late.
References
The Editors of Encyclopedia Britannica. “Influenza pandemic of 1918-19” Encyclopedia Britannica. Encyclopedia Britannica, inc. December 06, 2019. https://www.britannica.com/event/influenza-pandemic-of-1918-1919.
Center for Disease Control and Prevention. “Influenza (Flu)” December 2, 2019. https://www.cdc.gov/flu/prevent/keyfacts.htm