The 1918 flu pandemic (January 1918 – December 1920) was an unusually deadly influenza pandemic,
the first of the two pandemics involving H1N1 influenza virus (second one was Swine or Pig flu which broke out in 2009 and caused 17 000 deaths). However, Spanish flue was much more dangerous. It infected 500 million people across the world, including remote Pacific islands and the Arctic, and killed 50 to 100 million of them—three to five percent of the world’s population —making it one of the deadliest natural disasters in human history.
The war was over, but most horrible things were yet to come. Most influenza outbreaks disproportionately kill juvenile, elderly, or already weakened patients; in contrast the 1918 pandemic predominantly killed previously healthy young adults. First officially recorded influenza victims (on large numbers) were 100 American soldiers.
Some people might get confused by the name of pandemic. No, it did not start in Spain, however, due to war, America, Germany and France decreased the numbers of sick people in media. Spain was neutral, and media were free to inform society about horrible pandemic which spread all over the world.
The deadliest was the second wave of flu. First one was typical flu, and victims were usually elder people, however, the second one in August, when it began in France, Sierra Leone and the United States, flu had mutated in much deadlier version, which took away young, healthy people.
Those who get very ill stay home, and those mildly ill continue with their lives, preferentially spreading the mild strain. In the trenches, natural selection was reversed. Soldiers with a mild strain stayed where they were, while the severely ill were sent on crowded trains to crowded field hospitals, spreading the deadlier virus. The second wave began and the flu quickly spread around the world again.
After the lethal second wave struck in late 1918, new cases dropped abruptly – almost to nothing after the peak in the second wave.