[10] Scientists offer several explanations for the high mortality, including a six-year climate anomaly affecting migration of disease vectors with increased likelihood of spread through bodies of water.

[12] Malnourishment, overcrowded medical camps and hospitals, and poor hygiene, exacerbated by the war, promoted bacterial superinfection, killing most of the victims after a typically prolonged death bed.

[55] In an October 1918 "Madrid Letter" to the Journal of the American Medical Association, a Spanish official protested, "we were surprised to learn that the disease was making ravages in other countries, and that people there were calling it the 'Spanish grip'.

[70][71] World Health Organization 'best practices' first published in 2015 now aim to prevent social stigma by no longer associating culturally significant names with new diseases, listing "Spanish flu" under "examples to be avoided".

However, 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.

[101] 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.

[113] Michigan, for example, experienced a swift resurgence of influenza that reached its peak in December, possibly as a result of the lifting of the ban on public gatherings.

[124] 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.

[151] The Public Health Service announced it would take steps to "localize the epidemic",[152] but the disease was already causing a simultaneous outbreak in Kansas City and quickly spread outward from the center of the country in no clear direction.

[171] In 1977, an influenza virus bearing a very close resemblance to the seasonal H1N1, which had not been seen since the 1950s, appeared in Russia and subsequently initiated a "technical" pandemic that principally affected those 26 years of age and younger.

[83] A 2018 study of tissue slides and medical reports led by evolutionary biology 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.

[178] The major U.K. 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.

[182][183] 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.

He found archival evidence that a respiratory illness that struck northern China (where the laborers came from) in November 1917 was identified a year later by Chinese health officials as identical to the Spanish flu.

However, John Barry stated in his 2004 book The Great Influenza: The Epic Story of the Deadliest Plague In History that researchers have found no evidence to support this position.

One notable example was that of ice hockey player Joe Hall, who, while playing for the Montreal Canadiens, fell victim to the flu in April after an outbreak that resulted in the cancellation of the 1919 Stanley Cup Finals.

[209] Modern analysis has shown the virus to be particularly deadly because in animal trials it triggers an overreaction of the body's immune system (sometimes referred to as a cytokine storm).

[214] Studies have shown that the immune system of Spanish flu victims could have been weakened by adverse climate conditions which were particularly unseasonably cold and wet for extended periods of time during the duration of the pandemic.

Several explanations have been proposed for this, including the fact that lower temperatures and increased precipitation provided ideal conditions for virus replication and transmission, while also negatively affecting the immune systems of soldiers and other people exposed to the inclement weather, a factor proven to increase likelihood of infection by both viruses and pneumococcal co-morbid infections documented to have affected a large percentage of pandemic victims (one fifth of them, with a 36% mortality rate).

[228] Social distancing measures were introduced, for example closing schools, theatres, and places of worship, limiting public transportation, and banning mass gatherings.

[4] A 2006 study in The Lancet also noted that Indian provinces had excess mortality rates ranging from 2.1% to 7.8%, stating: "Commentators at the time attributed this huge variation to differences in nutritional status and diurnal fluctuations in temperature.

[279] The disease spread fastest through the higher social classes among the indigenous peoples, because of the custom of gathering oral tradition from chiefs on their deathbeds; many community elders were infected through this process.

[291] Estimates for the death toll in China have varied widely,[292][98] a range which reflects the lack of centralized collection of health data at the time due to the Warlord period.

These data were collected by the Chinese Maritime Customs Service, which was largely staffed by non-Chinese foreigners, such as the British, French, and other European colonial officials in China.

[177] However, this label has been challenged by the historian Guy Beiner, who has charted a complex history of social and cultural forgetting, demonstrating how the pandemic was overshadowed by the commemoration of the First World War and mostly neglected in mainstream historiography, yet was remembered in private and local traditions across the globe.

The general population was familiar with patterns of pandemic disease in the late 19th and early 20th centuries: typhoid, yellow fever, diphtheria, and cholera all occurred near the same time.

[363] These were subsequently used to experimentally infect mice, ferrets, and macaques giving valuable insights into influenza virus biology and pathogenesis, providing important information about how to prevent and control future pandemics.

[364] On 18 January 2007, Kobasa et al. (2007) reported that monkeys (Macaca fascicularis) infected with the recreated flu strain exhibited classic symptoms of the 1918 pandemic, and died from an overreaction of the immune system.

[371] In 2018, Michael Worobey, an evolutionary biology professor at the University of Arizona who is examining the history of the 1918 pandemic, revealed that he obtained tissue slides created by William Rolland, a physician who reported on a respiratory illness likely to be the virus while a pathologist in the British military during World War One.

Newfoundland's leading cause of death before the pandemic was tuberculosis and this is known to be a severe underlying condition for people and increases the |mortality rate when infected by the influenza disease.