The 1918 flu pandemic (the "Spanish flu") was one of the famous influenza pandemics in history. It was an unusually deadly and severe pandemic that spread across the world. This influenza pandemic was caused by a vicious Influenza A virus strain of subtype H1N1. Most victims were healthy young adults, in contrast to most influenza outbreaks, which predominantly affect juvenile, elderly, or weakened patients. The Spanish flu was implicated in the outbreak of encephalitis lethargica in the 1920s. The pandemic lasted from January 1918 to December 1920, spreading even to the Arctic and remote Pacific islands. Between 20 and 50 million died, making it one of the deadliest natural disasters in human history. The Spanish flu (1918 pandemic influenza) H1N1 Hemagglutinin (HA) proteins and antibodies were the main research tools for this influenza pandemic.
World War I did not cause the flu, but the close troop quarters and massive troop movements hastened the pandemic, and probably both increased transmission and augmented mutation; it may also have increased the lethality of the virus. Some speculate the soldiers' immune systems were weakened by malnourishment, as well as the stresses of combat and chemical attacks, increasing their susceptibility. A large factor in the worldwide occurrence of this flu was increased travel. Modern transportation systems made it easier for soldiers, sailors, and civilian travelers to spread the disease.
In the United States, the disease was first observed at Haskell County, Kansas, in January 1918, prompting local doctor Loring Miner to warn the U.S. Public Health Service's academic journal. On 4 March 1918, company cook Albert Gitchell reported sick at Fort Riley, Kansas. By noon on 11 March 1918, over 100 soldiers were in the hospital. Within days, 522 men at the camp had reported sick. By 11 March 1918 the virus had reached Queens, New York.
In August 1918, a more virulent strain appeared simultaneously in Brest, Brittany-France, in Freetown, Sierra Leone, and in the U.S. in Boston, Massachusetts. The Allies of World War I came to call it the Spanish flu, primarily because the pandemic received greater press attention after it moved from France to Spain in November 1918. Spain was not involved in the war and had not imposed wartime censorship.
Symptoms in 1918 influenza pandemic were of extreme severity, suspected to be caused by cytokine storms, and the symptoms were so unusual that initially influenza was misdiagnosed as dengue, cholera, or typhoid. One observer wrote, "One of the most striking of the complications was hemorrhage from mucous membranes, especially from the nose, stomach, and intestine. Bleeding from the ears and petechial hemorrhages in the skin also occurred." The majority of deaths were from bacterial pneumonia, a secondary infection caused by influenza, but the virus also killed people directly, causing massive hemorrhages and edema in the lung.
The global mortality rate from the 1918/1919 pandemic is not known, but an estimated 10% to 20% of those who were infected died. With about a third of the world population infected, this case-fatality ratio means 3% to 6% of the entire global population died. Influenza may have killed as many as 25 million people in its first 25 weeks. Older estimates say it killed 40–50 million people, while current estimates say 50–100 million people worldwide were killed.
The disease killed in every corner of the globe. As many as 17 million died in India, about 5% of the population. The death toll in India's British-ruled districts alone was 13.88 million. In Japan, 23 million people were affected, and 390,000 died. In the Dutch East Indies (now Indonesia), 1.5 million were assumed to have died from 30 million inhabitants. In Tahiti, 14% of the population died during only two months. Similarly, in Samoa after November 1918, 20% of the population of 38,000 died within two months. In the U.S., about 28% of the population suffered, and 500,000 to 675,000 died. Native American tribes were particularly hard hit. In the Four Corners area alone, 3,293 deaths were registered among Native Americans. Entire villages perished in Alaska. In Canada 50,000 died. In Britain, as many as 250,000 died; in France, more than 400,000. In West Africa, an influenza epidemic killed at least 100,000 people in Ghana. Tafari Makonnen (the future Haile Selassie, Emperor of Ethiopia) was one of the first Ethiopians who contracted influenza but survived, although many of his subjects did not; estimates for the fatalities in the capital city, Addis Ababa, range from 5,000 to 10,000, or higher. In British Somaliland one official estimated that 7% of the native population died.