The influenza pandemic between 1918 and 1919, more commonly known as the Spanish flu, was the biggest and most devastating pandemic of the 20th century. In fact, it was one of the worst so far in human history. The virus killed and spread very quickly. Not much was known about it at the time, which scared many people.
The pandemic hit in three different waves, the first during World War I. It spread through the western part of Europe, reaching Poland around July. This was a very early type of influenza, a mild one. It reached its stronger, more deadly, form during the second wave, which hit around August. This version developed more quickly, and many people developed pneumonia after just two days from symptoms starting. The third wave came during the wintertime. It concluded in the spring of 1919. Many 20 to 40-year-olds died during the final two waves, which was an unusual mortality age range for influenza.
The outbreaks of the flu were so vast that it reached almost every inhabited part of the world at the time. India was hit especially hard, where it is believed that at least 12 million people died. In the United States, there were roughly 550,000 deaths. The pandemic peaked during 1918-1919, and while there were several other outbreaks of the Spanish flu during the 1920s, they were not as severe.
An estimated 25 million people died from the Spanish flu, but some researchers believe there were as many as 40 to 50 million deaths worldwide. The influenza pandemic of 1918 was and still is considered to be one of the worst pandemics in human history.