Reports on Public Health and Medical Subjects, No. 4. The most widespread and serious pandemic of influenza occurred in 1918-19. It spread throughout Europe, Russia, Canada, S. America, New Zealand, Australia, Africa, India, China, and Japan. According to the Centers for Disease Control and Prevention,

"The 1918 influenza pandemic was the most severe pandemic in recent history. It was caused by an H1N1 virus with genes of avian origin. Although there is not universal consensus regarding where the virus originated, it spread worldwide during 1918-1919.  In the United States, it was first identified in military personnel in spring 1918. It is estimated that about 500 million people or one-third of the world’s population became infected with this virus. The number of deaths was estimated to be at least 50 million worldwide with about 675,000 occurring in the United States" (https://www.cdc.gov/flu/pandemic-resources/1918-pandemic-h1n1.html, accessed 3-2020).

Digital facsimile from the Hathi Trust at this link.