What Is Spanish Flu?


The Spanish Flu was a subtype of influenza A that caused a devastating pandemic during World War I, taking the lives of over 20 million people throughout the world. Despite its name, the Spanish Flu is a form of influenza A virus, which naturally infects wild aquatic birds as well as humans.

During the early 20th century, influenza spread through Europe, specifically in France, Spain, Britain and Italy. As it emerged in 1918-1919, during World War I, it was estimated that 500 million people were infected worldwide, resulting in 20-50 million deaths - making it one of the worst epidemics in history. The virus killed 2-20% of those infected, making it much more deadly than previous epidemics.1 The effects of this pandemic were exacerbated by the lower populations at the time, with the global population being only 28% of what it is today.1 As such, it is vital to understand both how the Spanish Flu pandemic emerged and how governments prepared for and responded to it in order to better prepare ourselves for future outbreaks.

Origins of the spanish flu

It is thought that the Spanish Flu emerged from poultry farms in the United States in 1917, with the first recorded case appearing in Fort Riley, Kansas, on the 11th of March 1918. Against the advice of many, soldiers living close to Fort Riley were shipped to Europe soon afterwards. Living and fighting in close quarters sparked and significantly accelerated the spread of the disease across Europe.2 Surprisingly, the disease killed more young and healthy individuals between the ages of 20 to 40 than those in other groups. This was largely due to the virus’s ability to trigger a strong immune response. Indeed, 60% of flu victims in South African cities were between 20 and 40 years of age.3  

How was the spanish flu identified?

When the virus first started to spread, British doctors did not believe that it was influenza due to its initial mildness. However, during the second wave of the pandemic, the Spanish Flu virus became much more virulent, to the point that it was frequently mistaken for cholera and typhoid.3

Previously, the laboratory methods used to identify tuberculosis and cholera were ineffective for influenza, as there was little understanding of this virus. Therefore, healthcare workers were unable to diagnose the Spanish Flu accurately.4 Today, the flu is characterised as H1N1, which was sequenced and analysed as an avian virus strain.1 H1N1 influenza is a subtype of influenza A and originated in birds.5 The types of influenza viruses are shown in the table below. Influenza subtypes (such as H1N1 here) emerge due to two processes known as antigenic drift and antigenic shift. These processes enable viruses to alter the proteins they express on their surface - and are the reason we need new flu vaccines every year.6,7 In 1997, the avian influenza virus H5N1 crossed the avian-human species barrier for the first time, causing six deaths in Hong Kong. Even today, H5N1 is endemic in countries such as Egypt, India, China, Vietnam, Indonesia, and Bangladesh.

Table 1: Influenza viruses belong to the family Orthomyxoviridae, and they are distinguished by an enveloped RNA. The table illustrates the type of influenza species. 8

TypeHostDiseaseOther hosts 
AAquatic birds (wild)Mild-severe symptoms, can be frequentDogs, cats, pigs, horses, poultry, humans, tigers 
BHumansMild symptoms and can be frequentSeals
CHumansRare but can range from mild-severe symptomsPigs
DCattleMild Pigs

Symptoms and transmission

Over time, the Spanish Flu pandemic provided public health experts insight into how the virus spread via respiratory transmission and close contact, as well as its characteristic symptoms.10 The most severe symptom of influenza A was pneumonia, which ultimately led to millions of deaths during World War I. The modes of transmission and symptoms of Spanish Flu are summarised in the table below:4

Modes of transmissionSigns and symptoms 
SneezingCoughingSpittingClose contactDropletsHigh temperature (up to 40 degrees)Fever and chillsPneumoniaMuscular achesNasal haemorrhage Dry coughDeath

Controversy surrounding the name ‘spanish flu’

There have been several controversies surrounding the name of the virus. Whilst the virus was initially called the ‘Spanish flu’ or ‘Spanish lady’, it did not actually originate in Spain.11 However, with Spain being a neutral country with free media during wartime, many people believed that all sources of the flu were originating from Spain - rather than America, as scientists now believe.

More than 100 years later, stereotypes still influence discussions about emerging viruses. For example, in 2019, when Sars-CoV-2 emerged, terms such as ‘Chinavirus’ circulated, bringing negative attitudes to the East Asian community. When a disease is named, it incorporates the origin, which can affect certain communities. However, the World Health Organisation (WHO) is now advocating for nonexclusive names.12

Responses to the spanish flu

How did the virus spread as extensively as it did? As the influenza pandemic was occurring during the First World War, political and social decisions intervened with (and often overpowered) the advice of scientific experts. Additionally, this was one of the first worldwide pandemics - meaning that effective and universally adopted response measures had yet to be put in place. As such, public health officials often prioritised the transportation of soldiers. Indeed, during the Armistice Day celebrations, many civilians were in close contact with soldiers, even hugging and kissing them - which spread the virus even further.13 However, when the virus reappeared in 1928-1929, the response was strategic, and quarantine measures were imposed to control the virus more effectively.4

Vaccine development

As previously mentioned, the Spanish flu was caused by the H1N1 Influenza A virus. There were no vaccines, drugs, or antibiotics available at the time of the pandemic; however, significant advancements in influenza research have been made since. Most notable is the development of the 2009 H1N1 (swine flu) vaccine.9 Although there is still no vaccine specific for the Spanish Flu, many lessons were learned from the pandemic that contribute to the development of further influenza treatments and vaccines. Furthermore, the development of the vaccine streamlined future vaccine production - enabling researchers to later produce  Sars-CoV-2 vaccines extremely quickly.14

It is recommended that you should take a flu vaccine yearly, especially if you are vulnerable or have a weak immune system. If you are living in the UK, you can book your vaccine here:



The Spanish flu taught us many lessons about how to respond to potential outbreaks and avoid worldwide pandemics - and demonstrated how science and politics must work together to save lives. Influenza viruses are unpredictable and constantly evolving and still infect 5-14% of the global population every year - highlighting the importance of stronger disease surveillance and development of new vaccine technology.15 Going forward, further research into H5N1 (and other influenza A viruses) is needed, as their mechanisms of infection and disease are still not fully understood. 


  1. Taubenberger JK. The origin and virulence of the 1918 “Spanish” influenza virus. Proc Am Philos Soc [Internet]. 2006 [cited Oct 14 2023];150:86–112. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2720273/
  2. Barry JM. The great influenza: the story of the deadliest pandemic in history. Revised ed. Penguin Books Limited; 2020. p418. 
  3. Knobler SL, Mack A, Mahmoud A, Lemon SM. The story of influenza. In: The Threat of Pandemic Influenza: Are We Ready? Workshop Summary [Internet]. National Academies Press (US); 2005 [cited Oct 14 2023]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK22148/
  4. Tomes N. “Destroyer and teacher”: managing the masses during the 1918–1919 influenza pandemic. Public Health Rep [Internet]. 2010 [cited Oct 14 2023];125:48–62. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2862334/
  5. Jilani TN, Jamil RT, Siddiqui AH. H1n1 influenza. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 [cited Oct 14 02023]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK513241/
  6. Korteweg C, Gu J. Pathology, molecular biology, and pathogenesis of avian influenza a (H5n1) infection in humans. Am J Pathol [Internet]. 2008 [cited Oct 14 2023];172:1155–70. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2329826/
  7. Wikramaratna PS, Sandeman M, Recker M, Gupta S. The antigenic evolution of influenza: drift or thrift? Philos Trans R Soc Lond B Biol Sci [Internet]. 2013 [cited Oct 14 2023];368:20120200. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3678325/
  8. Kaplan BS, Webby RJ. The avian and mammalian host range of highly pathogenic avian H5N1 influenza. Virus Res [Internet]. 2013 [cited Oct 14 2023];178(1):3–11. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3922066/
  9. Broadbent AJ, Subbarao K. Influenza virus vaccines: lessons from the 2009 H1N1 pandemic. Curr Opin Virol [Internet]. 2011 [cited Oct 14 2023];1:254–62. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3224079/
  10. Martini M, Gazzaniga V, Bragazzi NL, Barberis I. The Spanish Influenza Pandemic: a lesson from history 100 years after 1918. J Prev Med Hyg [Internet]. 2019 [cited Oct 14 2023];60:E64–7. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6477554/
  11. Antoni T, Guillem T, Carolyn Daer. The 1918 ‘“Spanish Flu”’ in Spain Clin. Infect. Dis [Internet]. 2008 [cited Oct 14 2023];47:987. Available from: https://academic.oup.com/cid/article/47/5/668/296225
  12. American Society for Microbiology. Why scientists should not name diseases based on location [Internet]. 2021 [cited Oct 14 2023]. Available from: https://asm.org:443/Articles/2021/May/Why-Scientists-Should-Not-Name-Diseases-After-Plac
  13. Wever PC, van Bergen L. Death from 1918 pandemic influenza during the First World War: a perspective from personal and anecdotal evidence. Influenza Other Respir Viruses [Internet]. 2014 [cited Oct 14 2023];8:538–46. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4181817/
  14. Events Institute of Medicine (US) Forum on Medical and Public Health Preparedness for Catastrophic Events. Vaccine supply. In: The 2009 H1N1 Influenza Vaccination Campaign: Summary of a Workshop Series [Internet]. National Academies Press (US); 2010 [cited Oct 14 2023]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK54181/
  15. Petrova VN, Russell CA. The evolution of seasonal influenza viruses. Nat Rev Microbiol [Internet]. 2018 [cited Oct 14 2023];16(1):47–60. Available from: https://www.nature.com/articles/nrmicro.2017.118
This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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Maysaah Seif Suleiman

Bachelors of Science Microbiology, BSc Microbiology, University of Reading

I am a recent Microbiology graduate. I have had the opportunity to contribute to two research projects during my time as an undergraduate; in vitro lung models to assess antimicrobial drug activities and discover a potential diagnostic tool for Mycobacterium bovis.

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