The Spanish Flu actually originated in China or Tibet in 1917, as a rare genetic shift of the influenza virus. A recombination of its surface proteins created a virus novel to almost everyone, thus creating an influenza without human immunity. Recently the virus has been reconstructed from the tissue of a dead soldier; it is now being genetically characterized.
The name of Spanish Flu came from the early infections and large mortalities in Spain, where it apparently killed 8 million in May 1918. However, an initial wave of influenza appeared early in the spring of 1918 in Kansas and in military camps throughout the U.S.
Before long cases were showing up in Europe. It changed its character when it hit France, becoming malignant as it was contracted by African soldiers who had been recruited into the French army.
After establishing a killing field in France, the flu moved into Spain--a neutral player in WWI. For that reason Spain had no need to censor the illness from its people in order to keep them focused on the war effort. The Spanish press therefore fully documented the illness, along with its destructive effects on the human body.
Running its course, the 1918 flu would affect a person in the following ways:
(1) High fevers, shivers, coughs, muscular pain and sore throat
(2) Tiredness and dizzy spells
(3) Loss of strength to the point of not being able to eat or drink without assistance
(4) Difficulty in breathing
The influenza epidemic quickly spread around the globe. In all, some 525 million people were infected by the virus, which caused about 21 million deaths. That was more than twice the number of individuals who had been killed by the Great War.
An unusual aspect of the 1918 flu virus was that, rather than behave like normal strains of influenza--which especially attacks those with weak immunity systems like the old, the infirm and the young--it tended to strike hardest at the young and healthy members of society. As these were the people who were responsible for the day-to-day organizational matters of people's lives, it sometimes must have seemed that society itself was crumbling.
In America strains of religious fervor swept the landscape, with "brimstone" preachers proclaiming such death was the sure hand of God smiting a sinful nation. Many soon turned to folk remedies of various sorts to avert the destruction that science at the time was simply unable to stop.
The Spanish Flu disappeared as quickly as it came, but only after killing millions of people worldwide. Re-appearing in March 1919, the world was better prepared, and the virus could be quarantined.
It infected 28% of all Americans, according to some estimates. Around 675,000 Americans died of influenza during the epidemic, ten times as many as in the "Great War". Influenza killed an estimated 43,000 American troops mobilized for WWI.
Half of the U.S. soldiers who died in Europe fell to the virus rather than the enemy, according to Deseret News.
As noted in the Journal of the American Medical Association edition of December 1918:
"The [year] 1918 has gone: a year momentous as the termination of the most cruel war in the annals of the human race...unfortunately a year in which [also] developed a most fatal infectious disease causing the death of hundreds of thousands of human beings. Medical science for four and one-half years devoted itself to putting men on the firing line and keeping them there. Now it must turn with its whole might to combating the greatest enemy of all--infectious disease."