Potential origins Despite its name, historical and
epidemiological data cannot identify the geographic origin of the Spanish flu. However, several theories have been proposed.
United States The first confirmed cases originated in the United States. Historian
Alfred W. Crosby stated in 2003 that the flu originated in
Kansas, and author
John M. Barry described a January 1918 outbreak in
Haskell County, Kansas, as the origin in his 2004 article. A 2018 study of tissue slides and medical reports led by professor Michael Worobey found evidence against the disease originating from Kansas, as those cases were milder and had fewer deaths compared to the infections in New York City in the same period. The study did find evidence through
phylogenetic analyses that the virus likely had a North American origin, though it was not conclusive. In addition, the
haemagglutinin glycoproteins of the virus suggest that it originated long before 1918, and other studies suggest that the reassortment of the H1N1 virus likely occurred in or around 1915.
Europe (1863–1944),
Self-Portrait with the Spanish Flu (1919) The major UK troop staging and hospital camp in
Étaples in France has been theorized by virologist
John Oxford as being at the center of the Spanish flu. According to Oxford, a similar outbreak occurred in March 1917 at army barracks in
Aldershot, A report published in 2016 in the
Journal of the Chinese Medical Association found evidence that the 1918 virus had been circulating in the European armies for months and possibly years before the 1918 pandemic. Political scientist
Andrew Price-Smith published data from the
Austrian archives suggesting the influenza began in Austria in early 1917. A 2009 study in
Influenza and Other Respiratory Viruses found that Spanish flu mortality simultaneously peaked within the two-month period of October and November 1918 in all fourteen European countries analyzed, which is inconsistent with the pattern that researchers would expect if the virus had originated somewhere in Europe and then spread outwards. Hannoun considered several alternative hypotheses of origin, such as Spain, Kansas, and Brest, as being possible, but not likely. No tissue samples have survived for modern comparison. Nevertheless, there were some reports of respiratory illness on the path the laborers took to get to Europe, which also passed through North America. China was one of the few regions of the world seemingly less affected by the Spanish flu pandemic, where several studies have documented a comparatively mild flu season in 1918. (This is disputed due to lack of data during the
Warlord Period.) This has led to speculation that the Spanish flu pandemic originated in China, as the lower mortality rates may be explained by the Chinese population's previously
acquired immunity to the flu virus. The pandemic is conventionally marked as having begun on 4 March 1918 with the recording of the case of Albert Gitchell, an army cook at
Camp Funston in Kansas despite there having been cases before him. The disease had already been observed away in
Haskell County as early as January 1918, prompting local doctor
Loring Miner to warn the editors of the
U.S. Public Health Service's journal
Public Health Reports. Within days of the 4 March case at Camp Funston, 522 men at the camp had reported sick. By 11 March 1918, the virus had reached
Queens, New York. Failure to take preventive measures in March/April was later criticized. As the U.S. had entered World War I, the disease quickly spread from Camp Funston, a major training ground for troops of the
American Expeditionary Forces, to other
U.S. Army camps and Europe, becoming an epidemic in the
Midwest,
East Coast, and French ports by April 1918, and reaching the
Western Front by mid-April. It then quickly spread to the rest of France, Great Britain, Italy, and Spain and in May reached
Wrocław and
Odessa. After the signing of the
Treaty of Brest-Litovsk (March 1918),
Germany started releasing Russian prisoners of war, who brought the disease to their country. It reached North Africa, India, and Japan in May, and soon after had likely gone around the world as there had been recorded cases in
Southeast Asia in April. In June an outbreak was reported in
China. After reaching Australia in July, the wave started to recede. The first wave lasted from the first quarter of 1918 and was relatively mild. Mortality rates were not appreciably above normal; in the United States ~75,000 flu-related deaths were reported in the first six months of 1918, compared to ~63,000 deaths during the same time period in 1915. In Madrid, Spain, fewer than 1,000 people died from influenza between May and June 1918. There were no reported quarantines; the first wave caused a significant disruption in the military operations of
World War I, with three-quarters of French troops, half the British forces, and over 900,000 German soldiers sick.
Deadly second wave of late 1918 flu patients at U.S. Army Camp Hospital no. 45 in
Aix-les-Bains, France, 1918 , playing a "tragic game of football" with a skeleton personification of the Spanish flu, November 1918 The second wave began in the second half of August 1918, probably spreading to
Boston, Massachusetts and
Freetown, Sierra Leone, by ships from
Brest, where it had likely arrived with American troops or French recruits for naval training. From the
Boston Navy Yard and
Camp Devens, about west of Boston, other U.S. military sites were soon afflicted, as were troops being transported to Europe. Helped by troop movements, it spread over the next two months to all of North America, and then to Central and
South America, also reaching Brazil and the Caribbean on ships. In July 1918, the
Ottoman Empire saw its first cases, in soldiers. From Freetown, the pandemic spread through West Africa along the coast, rivers, and railways, and from railheads to more remote communities, while South Africa received it in September on ships bringing back members of the
South African Native Labour Corps from France. From there it spread around southern Africa and beyond the
Zambezi, reaching Ethiopia in November. On 15 September, New York City saw its first fatality from influenza. The
Philadelphia Liberty Loans Parade, held in
Philadelphia, Pennsylvania, on 28 September 1918 to promote
government bonds for World War I, resulted in an outbreak causing 12,000 deaths. From Europe, the second wave swept through Russia in a southwest–northeast diagonal front, as well as being brought to
Arkhangelsk by the
North Russia intervention, and then spread throughout
Asia following the
Russian Civil War and the
Trans-Siberian railway, reaching Iran (where it spread through
Mashhad), and then India in September and China and Japan in October. The celebrations of the
Armistice of 11 November 1918 also caused outbreaks in
Lima and
Nairobi, but by December the wave was mostly over. The second wave of the 1918 pandemic was much more deadly than the first. The first wave had resembled typical flu epidemics; those most at risk were the sick and elderly, while younger, healthier people recovered easily. October 1918 was the month with the highest fatality rate of the whole pandemic. In the United States, ~292,000 deaths were reported between September–December 1918, compared to ~26,000 during the same time period in 1915. The
1918 flu pandemic in India was especially deadly, as Historian
David Arnold estimates at least 12 million dead, about 5% of the population.
Third wave of 1919 Pandemic activity persisted into 1919 in many places, possibly attributable to climate, specifically in the
Northern Hemisphere, where it was winter and thus the usual time for influenza activity. The pandemic nonetheless continued into 1919 largely independent of region and climate. with the Public Health Service issuing its first report of a "recrudescence of the disease" in "widely scattered localities" in early December. This resurgent activity varied across the country, however, possibly on account of differing restrictions. Pandemic interventions, such as bans on public gatherings and the closing of schools, were reimposed in many places in an attempt to suppress the spread. Significant outbreaks occurred in cities including
Los Angeles, New York City,
Memphis,
Nashville,
San Francisco, and
St. Louis. By 21 February, with some local variation, influenza activity was reported to have been declining since mid-January in all parts of the country. Following this "first great epidemic period" that had commenced in October 1918, deaths from pneumonia and influenza were "somewhat below average" in large U.S. cities between May 1919 and January 1920. Nonetheless, nearly 160,000 deaths were attributed to these causes in the first six months of 1919. It was not until later in the winter and into the spring that a clearer resurgence appeared in Europe. A significant third wave had developed in
England and Wales by mid-February, peaking in early March, though it did not fully subside until May. France also experienced a significant wave that peaked in February, alongside the Netherlands.
Norway,
Finland, and
Switzerland saw recrudescences of pandemic activity in March, and Sweden in April. Much of Spain was affected by "a substantial recrudescent wave" of influenza between January and April 1919. Portugal experienced a resurgence in pandemic activity that lasted from March to September 1919, with the greatest impact being felt on the west coast and in the north of the country; all districts were affected between April and May specifically. Influenza entered Australia for the first time in January 1919 after a strict maritime quarantine had shielded the country through 1918. It assumed epidemic proportions first in
Melbourne, peaking in mid-February. The flu soon appeared in neighboring
New South Wales and
South Australia. while a second, more severe wave occurred in
Victoria between April and June. In other parts, influenza recurred in the form of a true "third wave".
Hong Kong experienced another outbreak in June, as did South Africa during its fall and winter months in the
Southern Hemisphere.
New Zealand experienced some cases in May. Parts of South America experienced a resurgence of pandemic activity throughout 1919. A third wave hit Brazil between January and June.
Montevideo similarly experienced a second outbreak between July and September. The third wave particularly affected Spain,
Serbia,
Mexico and Great Britain, resulting in hundreds of thousands of deaths.
Fourth wave of 1920 In the Northern Hemisphere, fears of a "recurrence" of the flu grew as fall approached. Experts cited past flu epidemics, such as that of 1889–1890, to predict that such a recurrence a year later was not unlikely, though not all agreed. In September 1919,
U.S. Surgeon General Rupert Blue said a return of the flu later in the year would "probably, but by no means certainly," occur. France had readied a public information campaign before the end of the summer, and Britain began preparations in the autumn with the manufacture of vaccine. In Japan, the flu broke out again in December and spread rapidly throughout the country, a fact attributed at the time to cold weather. Pandemic-related measures were renewed to check the outbreak, and health authorities recommended the use of masks. Between October 1919 and 23 January 1920, 780,000 cases were reported across the country, with at least 20,000 deaths recorded by that date. This apparently reflected "a condition of severity three times greater than for the corresponding period of" 1918–1919. Despite its rapid peak at the beginning of the year, the outbreak persisted throughout the winter, before subsiding in the spring. In the United States, there were "almost continuously isolated or solitary cases" of flu throughout the spring and summer of 1919. An increase in scattered cases became apparent as early as September, but Chicago experienced one of the first major outbreaks of the flu beginning in the middle of January. The Public Health Service announced it would take steps to "localize the epidemic", but the disease was already causing a simultaneous outbreak in Kansas City and quickly spread outward from the center of the country. It became apparent within days of the start of Chicago's explosive growth in cases that the flu was spreading in the city at an even faster rate than in winter 1919, though fewer were dying. Within a week, new cases in the city had surpassed its peak during the 1919 wave. Around the same time, New York City began to see its own sudden increase in cases, and other cities around the country were soon to follow. Certain pandemic restrictions, such as the closing of schools and theaters and the staggering of business hours to avoid congestion, were reimposed in cities like Chicago, Memphis, and New York City. As they had during the epidemic in fall 1918, schools in New York City remained open, "An epidemic of considerable proportions marked the early months of 1920", the U.S. Mortality Statistics would later note; according to data at this time, the epidemic resulted in one third as many deaths as the 1918–1919 experience. New York City alone reported 6,374 deaths between December 1919 and April 1920, almost twice the number of the first wave in spring 1918. Poland experienced a devastating outbreak during the winter months, with its capital
Warsaw reaching a peak of 158 deaths in a single week, compared to the peak of 92 in December 1918; however, the 1920 epidemic passed in a matter of weeks, while the 1918–1919 wave had developed over the entire second half of 1918. By contrast, the outbreak in western Europe was considered "benign", with the age distribution of deaths beginning to take on that of
seasonal flu. A fourth wave also occurred in Brazil, in February.
Post-pandemic By mid-1920, the pandemic was largely considered to be "over" by the public as well as governments. Though parts of Chile experienced a third, milder wave between November 1920 and March 1921, The flu was also widespread in the United States, its prevalence in California reportedly greater in early March 1922 than at any point since the pandemic ended in 1920. In the years after 1920, the disease came to represent the "seasonal flu". The virus, H1N1, remained endemic, occasionally causing more severe or
otherwise notable outbreaks. The period since its initial appearance in 1918 has been termed a "pandemic era", in which all flu pandemics have been caused by its own descendants. Following the
first of these post-1918 pandemics, in 1957, the virus was totally displaced by the novel
H2N2, the
reassortant product of the human H1N1 and an
avian influenza virus, which thereafter became the active influenza A virus in humans. While some natural explanations, such as the virus remaining in some frozen state for 20 years, phenomenon, the nature of influenza itself has been cited in favor of human involvement of some kind, such as an accidental leak from a lab where the old virus had been preserved for research purposes. Following this miniature pandemic, the reemerged H1N1 became endemic again but did not displace the other active influenza A virus,
H3N2 (which itself had displaced H2N2 through a
pandemic in 1968). For the first time, two influenza A viruses were observed in cocirculation. This state has persisted even after 2009, when a
novel H1N1 virus emerged, sparked a
pandemic, and thereafter took the place of the seasonal H1N1 to circulate alongside H3N2. ==Epidemiology and pathology==