It infected 500 million people around the world, including remote Pacific islands and the Arctic, and resulted in the deaths of 50 to 100 million (three to five percent of the world's population), making it one of the deadliest natural disasters in human history.
Note: If you know your ancestry, you will probably find a relative who died from this pandemic, and 99.9% chance that all of your relatives at that time were sick with the influenza.
My grandfather, August Kusel's, first wife, Maggie McGrail, died October 20, 1918, from the influenza...their two children, Elsie and Herman were very ill but survived.
At the outbreak of the war, the United States pursued a policy of non-intervention, avoiding conflict while trying to broker a peace. When the German U-boat U-20 sank the British liner RMS Lusitania on 7 May 1915 with 128 Americans among the dead, President Woodrow Wilson insisted that "America is too proud to fight" but demanded an end to attacks on passenger ships. Germany complied.
In January 1917, Germany resumed unrestricted submarine warfare, realizing it would mean American entry. The German Foreign Minister, in the Zimmermann Telegram, invited Mexico to join the war as Germany's ally against the United States. In return, the Germans would finance Mexico's war and help it recover the territories of Texas, New Mexico, and Arizona.
The United Kingdom intercepted the message and presented it to the U.S. embassy in the U.K. From there it made its way to President Wilson who released the Zimmermann note to the public, and Americans saw it as casus belli. Wilson called on antiwar elements to end all wars, by winning this one and eliminating militarism from the globe. He argued that the war was so important that the U.S. had to have a voice in the peace conference. After the sinking of seven U.S. merchant ships by submarines and the publication of the Zimmermann telegram, Wilson called for war on Germany, which the U.S. Congress declared on 6 April 1917.