The Spanish flu outbreak 100 years ago is the modern world's deadliest epidemic, its toll of more than 50 million surpassing that of World War I.
Here is some background.
Why 'Spanish flu'?
Countries caught up in the 1914-1918 war censored information about the extent to which the flu outbreak was ravaging their troops.
Spain was, however, neutral in the conflict and had no such restrictions. Media reporting on the effects of 1918-1920 flu outbreak there resulted in the false impression that Spain was particularly hard hit, giving rise to the nickname.
Where did it come from?
The geographic origin of the epidemic is not certain and no samples remain for further study.
The first cases were recorded in March 1918 among soldiers in Kansas in the United States. It may have spread to Europe with the troops.
The virus is the type A(H1N1), the same strain behind the swine flu outbreak of 2009. That outbreak claimed about 18,500 lives in 214 countries, according to the World Health Organization (WHO), although later estimates say the final toll could be around 200,000.
It is believed that all existing type A flu viruses come directly or indirectly from the 1918 virus but are less harmful.
Spanish flu spread in three waves, the first in the Northern Hemisphere spring of 1918 being highly contagious but causing few deaths.
The second, more virulent, emerged later in the year and "swept the globe in six months, killing over 10,000 people per week in some US cities at the height of the pandemic," says a paper on the US National Academy of Sciences site.
"Emotional reports of fit and healthy soldiers falling down on parade and dying the same or the next day are recorded," another article says.
The third wave followed in early 1919.
Few regions were untouched. Australia was among the countries least affected because of its strict quarantine measures.
A ghastly toll
There is no precise death toll for the Spanish flu outbreak.
All estimates say that it caused many more deaths than World War I, when around 10 million soldiers were killed along with several million civilians.
The WHO regularly states "more than 50 million" died from the flu and one 2002 study, cited by many others since, says the toll may have been as high as 100 million.
About 500 million people, estimated at one-third of the world's population at the time, were infected, says a 2006 report entitled "1918 Influenza: the Mother of All Pandemics".
"Much of the high death rate can be attributed to crowding in military camps and urban environments, as well as poor nutrition and sanitation, which suffered during wartime," the Smithsonian Institute says.
The virulent form of the virus acted quickly, causing the lungs to fill with fluid and suffocating victims, some of whom died within days of their first symptoms.
While flu epidemics today tend to strike mainly young children and the elderly, the Spanish flu hit hardest among young adults aged between 20 and 40.
Its high-profile victims reportedly include: Austrian artist Egon Schiele (1918); French poet Guillaume Apollinaire (1918); Donald Trump's grandfather Frederick Trump (1918); Brazilian president Francisco de Paula Rodrigues Alves (1919).
The League of Nations, the forerunner of the United Nations, created in 1922 a health committee in part to respond to the need to fight such epidemics. It evolved into today's WHO.