The nations of the Allied side of World War I called it the "Spanish Flu" from mistaken early reports that it originated in Spain. In Spain it was called "The French Flu". Spain did have one of the worst early outbreaks of the disease, with some 8 million deaths in May of 1918.
Many infections with similar but milder symptoms were recorded in spring of 1918, with sore throat, headaches, dizzyness, and loss of appetite. The earliest known case was in Fort Riley, Kansas on March 11, 1918. In the summer infections became much more severe. In August of 1918 the more deadly version broke out simultaneously in three disparate locations - Brest, Boston and Freetown. Many of the worst outbreaks of the "Flu" were among soldiers, both at the front lines and in camps far away. Severe outbreaks often required hospitalization and even with the best of care often killing one third of those infected. The strain was unusual in commonly killing many young and healthy victims, as opposed to more common influenzas which caused the bulk of their mortality among the old and infirm. People without symptoms could be struck suddenly and be rendered too feeble to walk within hours; many would die the next day. Symptoms included a blue tint to the face and coughing up blood.
Global morbidity from the influenza was estimated at 2.5% of the population, with some 20% of the world suffering from the disease to some extent. The disease spread across the world killing twenty-five million in the course of six months; some estimates put the total of those killed world-wide at over twice that number, possibly as high as 70 million. An estimated 17 million died in India alone, with a morbidity of about 5% of the population. About 28% of the population of the USA suffered from the disease, and some 500,000 died from it. 200,000 were killed in England.
While it usually only infected less than 1/3 of the population in most places and killed only a fraction of those infected, there were a number of towns in several countries where the entire population was wiped out. The only sizable inhabited place with no documented outbreak of the flu in 1918-1919 was the island of Marajo at the mouth of the Amazon River in Brazil.
Many cities, states, and countries enforced restrictions on public gatherings and travel to try to stay the epidemic. In many places theaters, dance halls, and other public gathering places were shut down for over a year. Even in areas where morbidity was low, those incapacitated by the illness were often so numerous as to bring much of everyday life to a stop. There were many reports of places with no healthy health care workers to tend the sick and no able bodied grave diggers to inter the dead.
The Spanish Flu vanished within eighteen months, and the actual strain was never determined. The influenza virus was not understood by medical science at the time, and most contemporary effort was spent in an unsuccessful quest to find a germ-borne cause of the disease.
It has been suggested that the stresses of combat, possibly combined with the effects of chemical warfare, may have weakened soldiers' immune systems thereby increasing their vulnerability to the disease and accelerating its spread. If so then the epidemic might be seen as the last and most tragic outcome of the war.
In October 2002, The Armed Forces Institute of Pathology teamed up with a microbiologist from the Mount Sinai School of Medicine in New York. Together, they started to reconstruct the Spanish flu. In an experiment, published in October 2002, they were successful in creating a virus with two 1918 genes. This virus was much more deadly to mice than other constructs containing genes from contemporary influenza virus. The experiments were conducted under high biosafety conditions at a laboratory of the US Department of Agriculture in Athens, Georgia.
Source: Terrence M. Tumpey, Adolfo García-Sastre, Andrea Mikulasova, Jeffery K. Taubenberger, David E. Swayne, Peter Palese, and Christopher F. Basler Existing antivirals are effective against influenza viruses with genes from the 1918 pandemic virus. Proceedings of the National Academy of Sciences, 2002, 99: 13849-13854.