The influenza pandemic of 1918, known as the Spanish Flu, was one of the deadliest and widespread pandemics in human history. The scale of the outbreak, as well as limitations in technology, medicine and communication, create difficulties when trying to uncover accurate figures relating to the pandemic. Estimates suggest that the virus, known as the H1N1 influenza virus, infected more than one quarter of the global population, which equated to approximately 500 million people in 1920. It was responsible for roughly 25 million fatalities, although some projections suggest that it could have caused double this number of deaths. The exact origins of this strain of influenza remain unclear to this day, however it was first noticed in Western Europe in the latter stages of the First World War. Wartime censorship in Europe meant that the severity of the pandemic was under-reported, while news outlets in neutral Spain were free to report openly about the impact of the virus; this gave the illusion that the virus was particularly strong in Spain, giving way to the term "Spanish Flu".

Effects of the virus

By late summer 1918, the pandemic had spread across the entire continent, and the H1N1 virus had mutated into a deadlier strain that weakened the infected's immune system more than traditional influenzas. Some studies suggest that, in contrast to these traditional influenza viruses, having a stronger immune system was actually a liability in the case of the H1N1 virus as it triggered what is known as a "cytokine storm". This is where white blood cells release proteins called cytokines, which signal the body to attack the virus, in turn releasing more white blood cells which release more cytokines. This cycle over-works and greatly weakens the immune system, often giving way to other infections; most commonly pneumonia in the case of the Spanish Flu. For this reason, the Spanish Flu had an uncommonly high fatality rate among young adults, who are traditionally the healthiest group in society. Some theories for the disproportionate death-rate among young adults suggest that the elderly's immune systems benefitted from exposure to earlier influenza pandemics, such as the "Asiatic/Russian Flu" pandemic of 1889.

Decrease in life expectancy As the war in Europe came to an end, soldiers returning home brought the disease to all corners of the world, and the pandemic reached global proportions. Isolated and under-developed nations were especially vulnerable; particularly in Samoa, where almost one quarter of the population died within two months and life expectancy fell to just barely over one year for those born in 1918; this was due to the arrival of a passenger ship from New Zealand in November 1918, where the infected passengers were not quarantined on board, allowing the disease to spread rapidly. Other areas where life expectancy dropped below ten years for those born in 1918 were present-day Afghanistan, the Congo, Fiji, Guatemala, Kenya, Micronesia, Serbia, Tonga and Uganda. The British Raj, now Bangladesh, India and Pakistan, saw more fatalities than any other region, with as many as five percent of the entire population perishing as a result of the pandemic. The pandemic also had a high fatality rate among pregnant women and infants, and greatly impacted infant mortality rates across the world. There were several waves of the pandemic until late 1920, although they decreased in severity as time progressed, and none were as fatal as the outbreak in 1918. A new strain of the H1N1 influenza virus did re-emerge in 2009, and was colloquially known as "Swine Flu"; thankfully it had a much lower fatality rate due to medical advancements across the twentieth century.

From the mid-19th century until today, life expectancy at birth in the United States has roughly doubled, from 39.4 years in 1850 to 79.6 years in 2025. It is estimated that life expectancy in the U.S. began its upward trajectory in the 1880s, largely driven by the decline in infant and child mortality through factors such as vaccination programs, antibiotics, and other healthcare advancements. Improved food security and access to clean water, as well as general increases in living standards (such as better housing, education, and increased safety) also contributed to a rise in life expectancy across all age brackets. There were notable dips in life expectancy; with an eight year drop during the American Civil War in the 1860s, a seven year drop during the Spanish Flu empidemic in 1918, and a 2.5 year drop during the Covid-19 pandemic. There were also notable plateaus (and minor decreases) not due to major historical events, such as that of the 2010s, which has been attributed to a combination of factors such as unhealthy lifestyles, poor access to healthcare, poverty, and increased suicide rates, among others. However, despite the rate of progress slowing since the 1950s, most decades do see a general increase in the long term, and current UN projections predict that life expectancy at birth in the U.S. will increase by another nine years before the end of the century.

Life expectancy in the United Kingdom was below 39 years in the year 1765, and over the course of the next two and a half centuries, it is expected to have increased by more than double, to 81.1 by the year 2020. Although life expectancy has generally increased throughout the UK's history, there were several times where the rate deviated from its previous trajectory. These changes were the result of smallpox epidemics in the late eighteenth and early nineteenth centuries, new sanitary and medical advancements throughout time (such as compulsory vaccination), and the First world War and Spanish Flu epidemic in the 1910s.

In 1875, the average person born in Chile could expect to live to the age of 32 years, a figure that would remain largely stagnante throughout the late 19th and early 20th century, as the country’s Parliamentary era would see relatively little change in the day to day lives of the country’s citizens. Outside of two dips in 1910 and 1920, the latter primarily driven by the 1918 Spanish Flu epidemic. Life expectancy would see two sharp increases following the end of the First World War; the first in the 1920s, and the most dramatic in the early 1950s.

The first of these spikes, under President Ibáñez del Campo, can be attributed primarily to large increases in spending on public healthcare and improvements in public sanitation by the Campo administration. The second and larger spike, under President González Videla, can be attributed to a combination of mass immunization and vaccination, and the implementation of a national health care system, drastically cutting child mortality in the country. As a result of these reforms, life expectancy in Chile would more than double in just thirty years, rising from just over 33 years in 1925 to 69 years by 1955. Following the end of the Videla administration in 1952, life expectancy would continue to rise in Chile, as increasing urbanization, and the successful eradication of many childhood diseases would see both child and overall mortality decline. This rise has continued even into the 21st century, and as a result, life expectancy in Chile rose to over 78 years by the end of the century, and in 2020, it is estimated that the average person born in Chile will live to over 82 years old, the highest in South America.

A key question in ecology is the relative impact of internal nonlinear dynamics and external perturbations on the long-term trajectories of natural systems. Measles has been analyzed extensively as a paradigm for consumer-resource dynamics due to the oscillatory nature of the host-pathogen life cycle, the abundance of rich data to test theory, and public health relevance. The dynamics of measles in London, in particular, has acted as a prototypical test bed for such analysis using incidence data from the pre-vaccination era (1944–1967). However, during this timeframe there were few external large-scale perturbations, limiting an assessment of the relative impact of internal and extra demographic perturbations to the host population. Here, we extended the previous London analyses to include nearly a century of data that also contains four major demographic changes: the First and Second World Wars, the 1918 influenza pandemic, and the start of a measles mass vaccination program. By combining mortality and incidence data using particle filtering methods, we show that a simple stochastic epidemic model, with minimal historical specifications, can capture the nearly 100 years of dynamics including changes caused by each of the major perturbations. We show that the majority of dynamic changes are explainable by the internal nonlinear dynamics of the system, tuned by demographic changes. In addition, the 1918 influenza pandemic and World War II acted as extra perturbations to this basic epidemic oscillator. Our analysis underlines that long-term ecological and epidemiological dynamics can follow very simple rules, even in a non-stationary population subject to significant perturbations and major secular changes.

In 1870, the average person born in Colombia could expect to live to the age of 32 years old, a figure which would see only marginal increase in the country until the 1930s (with a minor decline in 1920 from the 1918 Spanish Flu epidemic). However, life expectancy would begin to increase rapidly starting in the 1930s, as a series of reforms and modernization programs following the Great Depression, combined with a relatively quick recovery from the financial crisis, would see significant improvements in standards of living and access to healthcare in the country. As a result, life expectancy would increase by nearly twenty years between 1930 and 1950 alone, and this would continue at an accelerated rate through much of the remainder of the century. While the economic and political instability resulting from the increased drug cartel and government clashes of the 1990s would see a pause in life expectancy growth, increases have largely resumed in the 21st century, and in 2020, it is estimated that the average person born in Columbia can expect to live to just over 77 years old.

Life expectancy in India was 25.4 in the year 1800, and over the course of the next 220 years, it has increased to almost 70. Between 1800 and 1920, life expectancy in India remained in the mid to low twenties, with the largest declines coming in the 1870s and 1910s; this was because of the Great Famine of 1876-1878, and the Spanish Flu Pandemic of 1918-1919, both of which were responsible for the deaths of up to six and seventeen million Indians respectively; as well as the presence of other endemic diseases in the region, such as smallpox. From 1920 onwards, India's life expectancy has consistently increased, but it is still below the global average.

In 1870, the average person born in Ecuador could expect to live to just under the age of 33. Outside of a temporary drop in the late 1910s, the result of the 1918 Spanish Flu epidemic, life expectancy in Ecuador would see only modest growth for much of the first half of the 20th century, reaching just over 35 years in 1940. However, life expectancy would begin to rise sharply in the 1940s, as reforms under President Galo Plaza Lasso would bring a period of stability and economic growth to the country, greatly improving standards of living and seeing the infant mortality rate drop dramatically. Between 1940 and 1950 alone, life expectancy would jump by more than twelve years, to almost 48 years.

Following the end of Galo Plaza’s term in 1952, Ecuador would endure a period of instability and civil unrest, causing life expectancy to temporarily stagnate for several years. However, life expectancy would begin to grow rapidly again starting in the 1960s as modernization would see a continuation of the improvements made in the 1940s. This growth would continue steadily into the 21st century, and in 2020, life expectancy from birth is almost 77 years.

In 1865, the average person born in Bangladesh could expect to live to just over the age of 25 years old. This figure would gradually decline throughout the remainder of the 19th century, falling to just over 20 years by 1900, as the Bengal region of British Raj would see several severe famines in this time period. However, life expectancy would begin to increase gradually in the early 1900s, outside of a small decline in the late 1910s from the 1918 Spanish Flu epidemic. Beginning in the 1930s, life expectancy in Bangladesh would begin to grow rapidly, as wartime exports and industrialization would lead to significant improvements in standards of living in the region. This growth would continue steadily throughout the rest of the century, declining only with the Bangladesh War of Independence in 1971. As access to and quality of healthcare in Bangladesh continuously improves, in 2020, the average person born in Bangladesh can expect to live to just over the age of 72 years old, which is higher than most other countries on the Indian subcontinent.

Life expectancy in Russia was 29.6 in the year 1845, and over the course of the next 175 years, it is expected to have increased to 72.3 years by 2020. Generally speaking, Russian life expectancy has increased over this 175 year period, however events such as the World Wars, Russian Revolution and a series of famines caused fluctuations before the mid-twentieth century, where the rate fluctuated sporadically. Between 1945 and 1950, Russian life expectancy more than doubled in this five year period, and it then proceeded to increase until the 1970s, when it then began to fall again. Between 1970 and 2005, the number fell from 68.5 to 65, before it then grew again in more recent years.

In 1875, those born in the present-day region of Indonesia lived to an averae age of thirty years. This figure would remain largely stagnant until the 1910s, where the the 1918 Spanish Flu epidemic would cause life expectancy to fall to just 27 years as the epidemic spread across the region. Following the end of the Spanish Flu epidemic in 1920, life expectancy would begin to rise in Indonesia, as public works and infrastructure programs by the Dutch colonial administration would see standards of living begin to rise in the country. However, after peaking at 36 years in 1940, life expectancy would fall to just thirty years once more with the invasion and subsequent occupation of the island by the Empire of Japan in 1942; most estimates suggest that between 2.4 and four million people in Indonesia died from famine, forced labor and violence during the Second World War.

Life expectancy would begin rising following the country’s independence from the Dutch in 1949, particularly in the early 1950s as mass immunization and vaccination, combined with rapid modernization would see child mortality and standards of living rise throughout the remainder of the century, reaching over 65 years by the turn of the millennium. This rise in life expectancy has continued in the 21st century, and in 2020, the average person born in Indonesia is expect to live to beyond the age of 71 years.

In 1870, it is estimated that life expectancy from birth in the area of modern-day Iraq was just over the age of 31 years. Life expectancy would remain largely stagnant at this level for much of the late 19th and early 20th centuries, interrupted only by a temporary dip in the late-1910s as the 1918 Spanish Flu epidemic would spread through the region. Life expectancy would begin to rise sharply in the years following the Second World War, however, as rising petroleum sales allowed a rapid modernization of the country, causing access to healthcare and standards of living to rise throughout the country. The large reductions in infant and child mortality were the driving force behind the increase in life expectancy.

This growth would continue steadily until the 1980s, when life expectancy would fall from just under 62 years in 1980, to 58.5 years in 1985, as the decade long Iran-Iraq War would lead to widespread fatalities and displacement in the country. As the fighting eased in the late 1980s, life expectancy began to rise once more in Iraq, reaching 69 years by the start of the 21st century. However, this growth would reverse once more in the 2000s, due to the U.S.-led invasion and occupation of Iraq in 2003, which in turn led to an insurgency of paramilitary and terrorist groups, and the subsequent civil war from 2011 to 2017. As Iraq continues its recovery following four decades of violence and instability, life expectancy is on the rise again, and in 2020, it is estimated that life expectancy from birth in Iraq is over seventy years.

Life expectancy in Algeria was approximately 29 years in 1845, and over the past 175 years it has increased to almost 77 years. It is assumed that a global influenza pandemic reduced life expectancy to 24 years in 1850, and the combination of famine and the fourth cholera pandemic caused Algerian life expectancy to fall below twenty years in 1870. The next decrease came after 1915, and while the First World War definitely contributed to the drop, the Spanish flu epidemic of 1918 (which was spread by the war) was the biggest contributor to lower life expectancy. During the Second World War, Algeria was controlled by the Axis powers until the Operation Torch that began late in 1942, when the allies started driving them away. During this time, Algeria's life expectancy fell from 36 to 34 years. Since 1950, Algeria's life expectancy has continuously increased, despite the Algerian War of Independence and Algerian Civil War. The largest increase in life expectancy came in the early 1980s, during a period of relative peace in Algeria, and in 2020, Algeria's life expectancy is more than double what it was before the Second World War.

In 1870, it is estimated that Kenyan life expectancy from birth was just 25.5 years. This low rate was in part the result of several famines and epidemics which ravaged the region throughout the late 1800s, including an epidemic in 1898, which, when combined with the coinciding famine, was estimated to have resulted in the death of over half the population of the country. The life expectancy would further drop in the late 1910s, the result of the 1918 Spanish Flu epidemic, which is estimated to have claimed the lives of over 5.5 percent of Kenya’s population.

Life expectancy would increase only marginally for much of the late 19th and early 20th centuries, but saw a significant increase in the years following the end of the Second World War. Kenyan life expectancy rose by almost ten years in the late 1940s. Life expectancy would continue to steadily rise for much of the 20th century, particularly so with the implementation of universal healthcare in 1965, before peaking at almost 59 years in 1985. However, beginning in the late-1980s, Kenya would see life expectancy fall significantly until the early 2010s, as the HIV/AIDS epidemic led to a significant increase in mortality across the population. After bottoming out at under 52 years in 2005, life expectancy was able to recover to pre-HIV/AIDS levels by the 2010s. In 2020, Kenya is estimated to have a life expectancy from birth of more than 66 years.

In 1870, the average child born in Peru could expect to live to just under the age of 36 years old. This figure would decline slowly throughout the late 19th and early 20th centuries, as internal and regional conflicts and economic instability would disrupt and hinder the advancement of Peruvian healthcare. The relatively sharp dip in the late 1910s can be attributed to the 1918 Spanish Flu epidemic. Life expectancy would begin to rise in the 1940s, however, a thriving economy and a series of populist policies implemented in this period would see living standards rise across much of the country.

Life expectancy has continuously grown since the 1940s, although at a slower rate than in the 1940s and 1950s. Healthcare advancements and mass immunization have been the driving forces behind Peru's increased life expectancy, due to the drastic decline in child mortality. This growth has continued into the 21st century, despite political and economic turmoil in recent years, and in 2020, it is estimated that the average person born in Peru will live to just over the age of 76 years.

In 1880, the average person born in the area of modern-day South Korea could expect to live to just under the age of 26, a figure which would remain below thirty until the 1920s. Life expectancy would fall to its lowest level of just 24 years in 1920, however, as the 1918 Spanish Flu epidemic would spread through the country, resulting in an estimated 200,000 deaths across the Korean peninsula. Life expectancy would begin to rise in the 1920s, however, as development programs by the Japanese colonial administration would see economic growth and access to healthcare improve greatly in the region. The 1940s and 1950s would see a slowing, then a reversal to this growth though, as the final years of the Second World War, and later the 1950 Korean War, would see significant destruction and fatalities in the country.

Following the end of the Korean War with the 1953 armistice, life expectancy would begin to climb again in the newly-established South Korea, as the country would begin to rapidly modernize and improve access to healthcare and nutrition, raising standards of living and cutting child mortality rates throughout the country. As a result, life expectancy would rise from just under 47 years in 1950, to over 75 years by the turn of the century. This rise in life expectancy has continued steadily into the 21st century, and as a result, in 2020, it is estimated that the average person born in South Korea will live to just under the age of 83 years, one of the highest life expectancies in the world.

In 1870, life expectancy from birth in the modern-day territory of Iran was approximately 25.6 years. This figure would see little change in Iran for much of the late 19th and early 20th century; the only major change was the dip in the late 1910s, resulting from the 1918 Spanish Flu epidemic) and a famine from 1918 to 1919, which led to significant fatalities across the region. Life expectancy would begin to grow rapidly in the 1940s, as the country’s oil reserves, discovered in 1908, would see an economic boom in the years during and immediately following the Second World War, providing a valuable source of funding for socio-economic reforms implemented under Reza Shah. This, in turn, saw a rapid modernization of healthcare in the country, as well as the implementation of mass vaccination programs in the early 1940s, which greatly lowered child mortality rates and allowed life expectancy to rise.

As implementation of these programs would continue, life expectancy from birth rose from just over 27 years in 1940, to approximately 39 years in 1950. After the end of the rapid growth of the 1940s, life expectancy would continue steadily rise until 1980, peaking at 56.7 years. However, life expectancy would fall to just 52 years in 1985, the result of mass fatalities in the Iran-Iraq War of 1980 to 1988. Following the war’s end with a UN-brokered ceasefire in 1988, life expectancy would quickly recover, reaching over 69 years by the turn of the century. Life expectancy has continued to rise throughout the 21st century, and it is now estimated that the average child born in Iran in 2020 will live to the age of approximately 76 years.

Life expectancy in Germany was below 39 in the year 1875, and over the course of the next 145 years, it is expected to have increased to above 81 years in the year 2020. Although life expectancy has generally increased throughout Germany's history, there were several times where the rate deviated from its previous trajectory. The most notable changes were because of the First and Second World Wars, in the first half of the twentieth century.

In 1910, life expectancy from birth in the region of present-day Syria was estimated at 32 years. This figure would see little change until 1935, outside of a brief dip in the late 1910s as the 1918 Spanish Flu epidemic would spread across the area. Life expectancy then began rising sharply from the late 1930s onwards, as modernization programs implemented by French, British and, later, independent authorities saw mass vaccination campaigns and healthcare improvements which greatly reduced child mortality rates. This climb would continue throughout the rest of the 20th century, and as a result, by 2000, the average person born in Syria could expect to live to over the age of 72.

However, life expectancy would see a sudden fall with the beginning of the Syrian Civil War in 2011. After peaking at just over 74 years in 2010, life expectancy would fall to just under 70 years by 2015, as the country’s civil war would lead to an estimated half a million fatalities, as well as mass displacement and the exodus of refugees from the country. As the fighting has gradually decreased in intensity in recent years, life expectancy of Syria has risen slightly, and in 2020, the average person born in Syria can expect to live to just over 71 years on average; although there is a difference of over eleven years in life expectancy between male and females.

Life expectancy in China was just 32 in the year 1850, and over the course of the next 170 years, it is expected to more than double to 76.6 years in 2020. Between 1850 and 1950, finding reliable data proved difficult for anthropologists, however some events, such as the Taiping Rebellion and Dungan Revolt in the nineteenth century did reduce life expectancy by a few years, and also the Chinese Civil War and Second World War in the first half of the twentieth century. In the second half of the 1900s, Chinese life expectancy increased greatly, as the country became more industrialized and the standard of living increased.