The Spanish flu (so-named because the Spanish press openly reported on the outbreak while other World War I belligerents suppressed the news) broke out in March 1918 at Camp Funston, an army camp in Kansas, and struck young, healthy adults with greater ferocity than any other demographic group. Occurring in three waves (March-September 1918, September-December 1918, Spring-Summer 1919) and affecting nearly 500 million people worldwide, the death toll reached 50 to 100 million people. The conditions of the First World War, poor hygiene, overcrowded hospitals, poor eating habits, and the general lack of knowledge and treatment for viral infections all contributed to its lethality.

The pandemic started in Haskell County, Kansas. Known both for its cattle and hog herds, it was also the flyway for 17 migratory bird flocks. It seems most likely that the birds infected the hogs, and the hogs infected humans. In January-February 1918, Dr. Loring Miner reported the initial cases to the United States Public Health Service. As carriers of the virus, recruits reported for military duty at Camp Funston spread the disease to others in the camp during March-April 1918.

In this early, springtime phase of the outbreak, despite the hundreds of cases, there were very few deaths. In April, soldiers from Camp Funston began the journey to France, while others were dispatched to other US military installations. The bases, transport trains and ships, and frontlines were crowded and unsanitary, creating ideal conditions for the spread of an airborne, infectious disease.

At the peak of the pandemic in October-November 1918, the mortality rate amongst soldiers on the Western Front reached 2%, while reports from India indicated a mortality rate of 10%. The third phase of the pandemic in 1919 was worsened by soldiers returning from the war through ports and military bases along the coast. These types of occurrences became familiar in port cities all around the world.

The virus that struck the world in 1918 to 1919 was the H1N1 influenza A variant. The virus originated in wild waterfowl, which transferred to hogs and pigs, then evolved to be transmitted to humans. Carried through the air from person to person, the virus overwhelmed the immune system by quickly replicating and infecting the upper respiratory system. In younger and stronger people, the immune system responded more intensely, causing severe inflammation and the build-up of fluids in the lungs. With the body weakened, the influenza infection most often led to secondary infections, especially pneumonia.

Upon exposure to the influenza virus, symptoms occurred within 2 to 3 days. The patient suffered from fever, chills, fatigue, muscle aches, headaches, cough, sore throat, nausea, and vomiting. Unique to the 1918 pandemic was the presence of a bluish tint to the skin (turning to purple near death), known as heliotrope cyanosis. This condition led to the lungs filling with fluid, causing the patient to suffocate. Death usually occurred within 3 to 5 days of the onset of the illness. The mortality rate was highest amongst the poor, soldiers, health workers, and people working in occupations characterized by crowded working conditions.

In major urban areas, crowded buildings and workplaces and the increasing number of people provided the virus with an abundance of hosts. The overcrowding was often accompanied by poor sanitation, poor water quality, insufficient food supply, and poor personal hygiene.

The war zones suffered from extreme overcrowding of soldiers and support personnel at the various military installations. The close living conditions at the camps, the conditions of trench warfare on WWI's Western Front, and troop ships helped the spread of the disease. Furthermore, poor sanitation, lack of proper nourishment, soldiers suffering from a variety of war wounds, and the low to non-existent natural immunity of the soldiers who had no previous exposure to the flu affected the spread. It was no wonder that the most affected demographic during the pandemic, especially in the fall of 1918, was the 20 to 40-year-olds, particularly soldiers and sailors.

Medical science would not make the distinction between bacteria and viruses until the 1930s, and it would not be until 2005 that the 1918 flu virus genome was finally mapped. Quinine treatments, especially in Italy, used in malaria hospital wards, seemed to show some effectiveness as few patients developed the flu, although opponents to the treatment argued the lack of scientific data to support quinine treatments, and malaria patients did get the flu. Aspirin was tried, but too often the doses administered were too high, leading to a build-up of fluids in the lungs and ultimately to death.

Another medicinal approach was mass vaccination. In England and the United States, doctors used a variety of vaccines, sometimes in combination with each other. The results of the vaccination campaigns were mixed. In 1918, the available vaccines were meant for bacterial infections, which caused some flu patients to become even sicker, while patients receiving a bacterial vaccine for pneumonia seemed to avoid death more successfully. Antibiotics were not available in 1918 to address secondary bacterial infections; the first antibiotic was only introduced after the discovery of penicillin in 1928.

Lacking any effective medical responses to the outbreak, communities relied on public gathering bans, including closures of private businesses, distancing, and masking. Closures included cinemas, theaters, and saloons/bars, however, it was not unusual for saloons to completely ignore orders. Public health officials filed lawsuits to compel businesses to comply, but court rulings were mixed. In some cases, courts ruled that the closure orders of health authorities did not apply, while other courts allowed the closures for a limited time only, as long as the epidemic was present in the community. As the pandemic dragged on, financial losses and job losses for businesses lessened their cooperation with officials.

The pandemic did not produce an economic crisis, although late 1918 and early 1919 experienced a mild recession due to the scaling back of war production. The flu most affected leisure and entertainment workplaces, but in 1918 to 1919, people had little money to spend on those activities. The war had already taught consumers the need to restrict their spending, so the addition of a pandemic had a negligible effect on consumption. Most people labored in industries or agriculture, and illness among workers particularly hurt coal, steel, textile, and copper producers. In some cases, those industries were unable to fill government orders necessary for the war effort. All the restrictions and limitations of conducting business and consumption were cast in patriotic terms, not only to combat the pandemic but to free up scarce resources for the war effort.

Public establishments were no less affected, especially schools and churches. Again, the record was mixed in regards to both the cooperation of local authorities and the effectiveness in combating the outbreak. Some clergy openly defied closure orders, while some congregations simply met in other places rather than their usual church buildings.

Recognized by doctors and other medical personnel as the only form of protection from the virus, masking ordinances were enacted in many places. Since the pandemic broke out during the war, public health campaigns encouraged people to wear a mask as a sign of their patriotic duty, but resistance to mask-wearing was high.

Public responses varied from place to place, largely at the state or city level. There was recognition that many of the responses would be costly in social, economic, and legal terms. People engaged in behaviors rooted in the idea that cleanliness would be an effective preventative against the illness. Face masks, disinfection, and frequent hand- and food washing were encouraged. There were attempts to sanitize public buildings and public transportation. Products in the cleanliness campaign included lime milk, soda lye, and steaming bedding and laundry. Cleaning and ventilating homes often was also advised.

People were recommended to avoid crowds both indoors and outdoors, as those were one of the greatest risk factors in the spread of the illness. In some places, bans were enacted, although many people ignored the measures, which led to an increased number of ill people and deaths. People were also recommended to stay out of public spaces, especially if they were showing symptoms of the flu or were infected. Isolating individuals and families at home helped to combat the spread.

Other well-meaning advice to combat the flu included eating simple, well-cooked meals, not spitting on floors, and avoiding strong air drafts. Of course, there were also recommendations that not only proved futile but were, in fact, quite silly, such as camphor ball necklaces, gargling, fumigation, carbolic spray, eating onions, and assembling in large gatherings in order to spread the germ so people might develop natural immunity to the disease.

The pandemic was felt across the globe. In Africa, the wave of Spanish Flu has been labeled the continent's worst ever short-term disaster. Hitting all of sub-Saharan Africa and entering through major ports like Mombasa, Cape Town, and Freetown, the virus spread inland via river transport, the movement of migrants, soldiers returning home, and the movement of labour for such projects as railway construction. Often faced with a complete lack of medical facilities, authorities at least encouraged social distancing, quarantines, and the closure of public meeting spaces like schools and religious buildings, as well as the suspension of basic services. Affecting all levels of society, between 1.5 and 2.5 million Africans perished.

It was a similar story elsewhere. In China, over 2.5 million people died, with some villages suffering a 10% mortality rate. However, it may be that Chinese people had a higher natural immunity to the virus, acquired from earlier waves of the flu. Traditional Chinese medicine may also have helped in limiting deaths in terms of preventative treatments.

India was subjected to the pandemic, the virus carried in by troop ships landing at Bombay (Mumbai) and Karachi. Again, general population movements spread the virus inland. The virus particularly struck 20 to 40 year-olds, the worst hit being the poor, those who lived in rural areas, and young women of reproductive age (there was a 30% drop in the birth rate as a consequence). With the country already suffering a famine, rural people moving to urban centres to find food only worsened the spread of the virus. Once again, medical facilities and supplies were found wanting, but mobile fever clinics at least helped feed and treat the sick. Estimates of the number of people killed by the virus range from 12 to 18.5 million, but it may have been well over 50 million in reality.

South and Central America were similarly hit. Most victims were in the 24 to 44 age bracket, and they included the president of Brazil. Mexico suffered the same three waves as the rest of the world. At one point, 1,500 to 2,000 people were dying each day. In a desperate response to the virus' spread, a sort of sanitary dictatorship was established, which limited political interference in the management of the medical emergency. People's movements were severely restricted, and a vast array of measures were enforced, ranging from curfews to the wearing of masks.

The pandemic ended by late spring in 1919. By that time, nearly one-third of the world's population had been infected, and roughly 3% of the world's population died (50 to 100 million people). Those who suffered the most were in the 25 to 40-year-old demographic. The influenza pandemic produced lasting consequences, apart from the number of people who fell ill or died, affecting public health measures, medicine – especially the emerging fields of epidemiology and virology – economics, art & literature, etc. No segment of human society was unaffected.

The pandemic produced a whole host of medical innovations. Doctors learned of the effectiveness of blood transfusions from survivors to the sick, which ushered in the era of blood typing to make the transfusions compatible. As scientists developed a better understanding of the virus, and disease in general, by the late 1920s/early 1930s, researchers, using the new electron microscope, developed techniques to view the virus. Simultaneously, scientists began growing the virus in chicken eggs, which allowed them to discover two types of influenza: A and B. By the late 1930s, British and American scientists began testing a new vaccine to combat influenza. In 1944, the vaccine was used on soldiers, and the following year witnessed the mass vaccination of civilians. Those early flu vaccines contained both types of viruses, which would result in the multi-virus vaccine in use today. The tools used to create these early flu vaccines were also used to develop vaccines for other infectious diseases. The research would enhance medical science's understanding of genes, leading to the cracking of the genetic code of human DNA in 1944.

Writers and artists during and in the post-WWI era reflected the suffering and death caused by the pandemic. Virginia Woolf's character, Clarissa Dalloway, in Mrs. Dalloway (1925), suffered from a heart condition caused by the flu. Anne Porter, author of Pale Horse, Pale Rider (1939), not only contracted the flu but her soldier-lover, who nursed her back to health, would die from the flu, which he contracted from her. T.S. Eliot, in 1922, wrote Wasteland, which described society in the aftermath of war and disease as a wounded land, both wasted and decapacitated. Ahmed Ali's Twilight in Delhi (1940) highlights the 1918 flu pandemic as a symbol of the ending of the old order, the economic disruption, and loss of memory that humans have for earlier pandemics, making them unprepared for the 1918 outbreak.

Artists depicted the themes of trauma and despair while themselves suffering from the illness. Gustav Klimt (1862 to 1918) died from a stroke caused by the flu. Edvard Munch (1863 to 1944) and John Singer Sargent (1856 to 1925) contracted the disease but survived. Sargent had been sent to the front by the British government to sketch the joint actions of British & American troops. In the post-war period, various art monuments reflected the theme of hopelessness and ways in which people tried to cope with war & disease. Dadaism, the Bauhaus, and the abstract art movement engaged in new forms, created more practical and useful objects, or departed from reality to go beyond literal ideas and emotions.

Lastly, in the realm of social sciences, the 1918 pandemic undermined the eugenic & social Darwinism movements. The disease struck everyone regardless of class, status, income, age, etc. The pandemic undermined the idea that there existed some group of 'superior' people in society. The flu undercut the notion that people were somehow responsible for catching diseases, either due to some character flaw or physical defect. No better example of this was the illness that struck the American president, Woodrow Wilson (1856 to 1924), who became ill while attending the Paris Peace Conference to sign the Treaty of Versailles in 1919. Wilson suffered from a fever, coughing fits, diarrhea, weakness, and confusion. Many writers have assumed that Wilson suffered a stroke while in Paris (a condition that struck him back in the United States while he tried to convince the US Congress and the American people to support the League of Nations), but in fact, Wilson caught the flu, shown by the symptoms he suffered from. The influenza pandemic swept the world, showing no favoritism toward any individuals, region, or nation, and became the deadliest pandemic of the 20th century.