Even in 2021, bubonic plague continues to exist in nature, and there are generally a few thousand human cases per year. Going back to the beginning of the 20th century, it is estimated that there were roughly one million cases per year in 1907. Within two decades, this number had fallen below one fifth of this level to 170,000 cases per year in the 1920s, and in the 1940s it was just over 20,000 per year. By the mid-20th century, it had fallen below 5,000 cases per year, but the rapid decrease in cases observed in the first half of the 1900s did not continue through the second half of the century. Even in 2019, there was one case of plague recorded in the United States. How infection occurs Yersinia pestis is the bacteria that causes the plague virus, and it is most commonly spread by rats and their fleas. The disease survives by fleas infecting rats, which in turn infect other fleas; the majority of rats survive the disease, which facilitates its spread; this is known as the "enzootic cycle ". Interestingly, the disease is usually fatal for the fleas, as it blocks their "stomachs" and causes them to starve; as the fleas get hungrier, they attempt to feed on more hosts, spreading the disease more rapidly. When the rats die, the parasitic fleas then search for a new host, which means that other animals (particularly mammals) are susceptible to this virus. While rat fleas can not survive on other hosts for very long, they can infect other (including human) fleas with the virus. The most common way for humans to contract the plague however, is when a rat flea bites its human host; during this process the flea simultaneously regurgitates Yersinia pestis bacteria into the wound, and this causes bubonic plague. Humans can then spread the disease among one another by coming into contact with the infected tissue or fluids of an infected person, or from the transfer of fleas. Continued existence of the plague Plague is extremely difficult to eradicate in nature, as rodent communities in the wild provide natural reservoirs for the disease to spread. In previous centuries, rats had much more frequent contact with humans for a variety of reasons; houses were more often made of wood (which made infestations easier), public spaces were much dirtier, and the presence of rats was tolerated more. As the understanding of epidemiology grew in the 20th century, this greatly reduced the frequency of plague in human populations. Unlike human diseases such as smallpox, which was eradicated through vaccination and other medical advancements, basic sanitation and the extermination of rats have been the driving force behind the decline of plague.

The Plague of Justinian was an outbreak of bubonic plague that ravaged the Mediterranean and its surrounding area, between 541 and 767CE. It was likely the first major outbreak of bubonic plague in Europe, and possibly the earliest pandemic to have been recorded reliably and with relative accuracy. Contemporary scholars described the symptoms and effects of the disease in detail, and these matched descriptions of the Black Death and Third Pandemic, leading most historians to believe that this was bubonic plague. It was also assumed that the plague originated in sub-Saharan Africa, before making its way along the Nile to Egypt, and then across the Mediterranean to Constantinople. In 2013, scientists were able to confirm that Justinian's Plague was in fact Yersinia pestis (the bacteria which causes bubonic plague), and recent theories suggest that the plague originated in the Eurasian Steppes, where the Black Death and Third Pandemic are also thought to have originated from, and that it was brought to Europe by the Hunnic Tribes of the sixth century. Plague of Justinian The pandemic itself takes its name from Emperor Justinian I, who ruled the Byzantine Empire (or Eastern Roman Empire) at the time of the outbreak, and who actually contracted the disease (although he survived). Reports suggest that Constantinople was the hardest hit city during the pandemic, and saw upwards of five thousand deaths per day during the most severe months. There are a multitude of sources with differing estimates for the plague's death toll, with most ranging between 25 and 100 million. Until recently, scholars assumed that the plague killed between one third and 40 percent of the world's population, with populations in infected regions declining by up to 25 percent in early years, and up to 60 percent over two centuries. The plague was felt strongest during the initial outbreak in Constantinople, however it remained in Europe for over two centuries, with the last reported cases in 767. Pre-2019 sources vary in their estimates, with some suggesting that up to half of the world's population died in the pandemic, while others state that it was just a quarter of the Mediterranean or European population; however most of them agree that the death toll was in the tens of millions. Historians have also argued about the plague's role in the fall of the Roman Empire, with opinions ranging from "fundamental" to "coincidental", although new evidence is more aligned with the latter theories. Challenging theories As with the recent studies which propose a different origin for the disease, one study conducted by researchers in Princeton and Jerusalem calls into question the accuracy of the death tolls estimated by historians in the 19th and 20th centuries. In 2019, L. Mordechai and M. Eisenberg published a series of papers suggesting that, although the plague devastated Constantinople, it did not have the same impact as the Black Death. The researchers argue that modern historians have taken a maximalist approach to the death tolls of the pandemic, and have applied the same models of distribution to Justinian's Plague as they believe occurred during the Black Death; however there is little evidence to support this. They examine the content and number of contemporary texts, as well archaeological, agricultural and genetic evidence which shows that the plague did spread across Europe, but did not seem to cause the same societal upheaval as the Black Death. It is likely that there will be further investigation into this outbreak in the following years, which may shed more light on the scale of this pandemic.

The Black Death was the largest and deadliest pandemic of Yersinia pestis recorded in human history, and likely the most infamous individual pandemic ever documented. The plague originated in the Eurasian Steppes, before moving with Mongol hordes to the Black Sea, where it was then brought by Italian merchants to the Mediterranean. From here, the Black Death then spread to almost all corners of Europe, the Middle East, and North Africa. While it was never endemic to these regions, it was constantly re-introduced via trade routes from Asia (such as the Silk Road), and plague was present in Western Europe until the seventeenth century, and the other regions until the nineteenth century. Impact on Europe In Europe, the major port cities and metropolitan areas were hit the hardest. The plague spread through south-western Europe, following the arrival of Italian galleys in Sicily, Genoa, Venice, and Marseilles, at the beginning of 1347. It is claimed that Venice, Florence, and Siena lost up to two thirds of their total population during epidemic's peak, while London, which was hit in 1348, is said to have lost at least half of its population. The plague then made its way around the west of Europe, and arrived in Germany and Scandinavia in 1348, before travelling along the Baltic coast to Russia by 1351 (although data relating to the death tolls east of Germany is scarce). Some areas of Europe remained untouched by the plague for decades; for example, plague did not arrive in Iceland until 1402, however it swept across the island with devastating effect, causing the population to drop from 120,000 to 40,000 within two years. Reliability While the Black Death affected three continents, there is little recorded evidence of its impact outside of Southern or Western Europe. In Europe, however, many sources conflict and contrast with one another, often giving death tolls exceeding the estimated population at the time (such as London, where the death toll is said to be three times larger than the total population). Therefore, the precise death tolls remain uncertain, and any figures given should be treated tentatively.

The Third Plague Epidemic began in the mid-1800s in Yunnan, China, (an area that is still a natural reservoir for the Yersinia pestis bacteria) and had a huge death toll across Asia in the next century. While plague was confined to the Yunnan region for some decades, the mass displacement and social upheaval caused by the Taiping Rebellion saw millions flee the area , bringing the disease to other parts of the country. A plague epidemic then emerged in British-controlled Hong Kong in 1894, where merchants then unknowingly transported infected rats to other parts of the empire along various trade routes. Arrival in Bombay The first Indian cases were reported in Bombay (present-day Mumbai), and the Bombay Presidency suffered more losses than any other region in India (although there were some individual years where the state of Punjab reported more deaths). As with most disease or famine outbreaks in the region, the British authorities were slow to react, and their eventual response was in many ways too late. In some cases authorities even facilitated the spread of the disease; with multiple accounts of the military forcing healthy people into quarantine camps, evicting and burning homes of the afflicted, or by using such excessive force that the public would refuse medical help. Spread in India Lack of understanding among the Indian public was also to their own detriment. Some religions in India forbid the killing of rats, while some people simply refused to acknowledge that they were sick. As the plague in Bombay spiraled out of control, many fled to other parts of the country, taking the plague with them. It is estimated that there were over one million deaths in India by 1902, and almost one million further deaths in 1903 alone. The first four months of 1904 also saw over half a million deaths, almost matching the entire total for 1902. Plague would remain endemic to India for the next few decades, and there are varying reports of up to 10 or 12 million total plague deaths in this time. The public health measures taken to combat the plague in the early 20th century would mark the beginnings of India's public health system, and some of the quarantine measures put in place by the colonial government were even used in 2020 during the outbreak of the COVID-19 pandemic.

The Hong Kong plague epidemic, which began in 1894, gave way to some of the most important discoveries in the field of epidemiology. Swiss-French scientist, Andre Yersin, successfully identified the plague-causing bacteria Yersinia pestis, and showed that this bacteria was also present in rodents during this time. Following this discovery, scientists then investigated the connection further, in order to establish the extent of the infection in rats. Results In these experiments, all of the rats tested carried the plague in their blood, and over 90 percent had it in their spleen. The four tests conducted had similar results in both regions, although the infections had typically spread further within the rats by June than it had in May. The significance of these infections was that each rat became more contagious as the disease spread to other parts of the body. For example, as all of these rats had contaminated blood, this meant that any fleas or other animals who fed on infected rats would likely contract the disease as a result; in contrast, only 15 to 34 percent of those studied had plague bacteria in their saliva, meaning that a bite from an infected rat would generally not transmit the plague. Plague today While human diseases such as smallpox and polio have been or are in the process of being eradicated through vaccination, the presence of plague in rat colonies makes it much more difficult to eradicate. While increased sanitation and control measures have made plague almost non-existent in most countries, it continues to be endemic to rural areas of sub-Saharan Africa, Madagascar, and Peru. Between 2010 and 2015, there were 3,248 cases and 584 deaths worldwide due to plague, mostly in the aforementioned areas. The reason for its continued existence is due difficulties in locating and exterminating the infected rat colonies which act as natural reservoirs for the disease. Nonetheless, while the number of cases and deaths has been very low for decades, these numbers continue to decrease and this trend is expected to continue into the future.

As early as 1319, allegations of well-poisoning had been levelled at leper communities in Europe, in an attempt to demonize and ostracize this group in society. In France and Spain in 1321, the "leper's plot" developed into a widespread conspiracy, claiming that leper communities were acting on the orders of the Jews or Spanish Moors, poisoning water supplies in an attempt to spread disease among Christians. Under royal decrees, many lepers were then tortured into confessing to these acts, and were subsequently burnt at the stake (although this was often carried out by vigilante mobs before it could be done by the courts). After the initial hysteria in 1321, the involvement of lepers was quickly dismissed, and a papal bull was introduced to grant protection to leper communities in France; this however did not dispel the myths surrounding the Jews' involvement in the conspiracy, and the issue emerged again a few decades later. Why the Jews were blamed When the bubonic plague made its way to Europe, many were eager to find a scapegoat on whom they could blame their misfortune. The "well-poisoning" accusations were quickly raised again against Jewish communities in France and Spain, and also across the German states. Historians point to several reasons why Jews were blamed for the Black Death; many Jews lived in separate communities and did not use the same common wells, and Jewish religious practices promote bathing and hand-washing; both of these factors meant that the plague spread differently and at a different rate among Jews than it did among the general population. Modern historians also point to the fact that Jews were often moneylenders, and their debtors often used the plague as an opportunity to expunge their debts; Holy Roman Emperor Charles IV also forfeited the property of Jews who were killed in the pogroms, giving further impetus to these mobs. Anti-Jewish pogroms The first reported pogroms took place in Toulon in 1348, before the violence then spread across the rest of Western Europe. Over the next three years, hundreds of Jewish communities were attacked and exterminated, with the majority taking place in the German states. A number of larger communities, such as those in Cologne and Mainz, were destroyed completely, resulting in the deaths and forced conversions of thousands of Jews. Pope Clement VI introduced two papal bulls in 1348, which granted the church's protection to Europe's Jews. He also urged the clergy and nobility to take measures that protected Jews in their local areas, although most sources show that authorities were apathetic or complicit in the actions of the mobs. There is even evidence that authorities orchestrated several of the pogroms, such as in Strasbourg, where authorities led the city's Jewish community to a newly-built house outside the city, but when they arrived, any Jews who refused to convert to Christianity were then burned alive inside the house. Legacy Many of the sources present different versions of events, with death tolls ranging from one hundred to several thousand in some cases, while some sources also claim that Jews set fire to their own homes rather than convert. It is now impossible to confirm the exact sequence of events, or the actual number of deaths resulting from these pogroms, however, the limited sources available do provide a brief foundation for the modern understanding of medieval anti-Semitism and the destruction inflicted upon the Jews during the plague. It is also important to note that these pogroms were not unique to the Black Death's outbreak, and there is evidence of numerous massacres of Jewish communities in the centuries that followed. The demographic impact of the massacres was that there was a mass exodus of Jews from west to Eastern Europe, to countries such as Poland (where they were actually welcomed by authorities). The consequences of this demographic shift would be most felt six centuries later, when millions of Jews across Eastern Europe were exterminated at the hands of the Nazi regime during the Holocaust.

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.

The Great Smallpox Pandemic of 1870 to 1875 was the last major smallpox epidemic to reach pandemic level across Europe. The outbreak has its origins in the Franco-Prussian War of 1870 to 1871, where unvaccinated French prisoners of war infected the German civilian population, before the virus then spread to all corners of Europe. The death rates peaked in different years for individual countries; with the highest numbers recorded in 1871 for the German states, Belgium and the Netherlands, while death rates peaked in Austria, Scotland and Sweden in later years (the states that peaked in 1871 were closer in proximity to the frontlines of the Franco-Prussian War). Impact of compulsory vaccination The average number of deaths per million people was much higher in countries without compulsory vaccination, ranging from 953 to 1,360 in the samples given here. In comparison to this, the countries with compulsory vaccination barely reached these numbers in the years when the epidemic was at its worst, and their annual averages ranged between 314 and 361 deaths per million people during the six years shown here. Impact of the Great Pandemic Following the surge in smallpox deaths caused by the pandemic, many of the countries listed here introduced mandatory vaccination, or introduced penalties for parents who did not vaccinate their children. Germany and the Netherlands** did this in 1874, while Britain and Sweden enforced their vaccination laws with stricter penalties in 1871 and 1880 respectively. Perhaps surprisingly, Austria and Belgium, the two countries with the highest average death rate shown here, never introduced mandatory smallpox vaccination.

Following the arrival of Spanish colonizers in 1519, namely Hernando Cortes and his 600 conquistadors, the indigenous population of the Mexican valley saw a dramatic decline. In the first two years of conquest, thousands of indigenous Americans perished while fighting the European invaders, including an estimated 100,000 who died of violence or starvation during Cortes' siege of the Aztec capital city, Tenochtitlan (present-day Mexico City), in 1520. However, the impact of European violence on population decline pales in comparison to the impact of Old World diseases, which saw the indigenous population of the region drop from roughly 22 million to less than two million within eight decades.. Virgin soil pandemics Almost immediately after the Spanish arrival, a wave of smallpox swept across the indigenous populations, with some estimates suggesting that five to eight million natives died in the subsequent pandemic between 1519 and 1520. This outbreak was not an isolated incident, with the entire indigenous population of the Americas dropping by roughly ninety percent in the next two centuries. The Mexican valley specifically, which was the most populous region of the pre-Columbian Americas, suffered greatly due to virgin soil pandemics (where new diseases are introduced to biologically defenseless populations). In the Middle Ages, the majority of Europeans contracted smallpox as children, which generally granted lifelong immunity. In contrast, indigenous Americans had never been exposed to these diseases, and their populations (of all ages) declined rapidly. Cocoliztli Roughly three decades after the smallpox pandemic, another pandemic swept across the valley, to a more devastating effect. This was an outbreak of cocoliztli, which almost wiped out the entire population, and was followed by a second pandemic three decades later. Until recently, historians were still unsure of the exact causes of cocoliztli, with most hypothesizing that it was a rodent-borne disease similar to plague or an extreme form of a haemorrhagic fever. In 2018, however, scientists in Jena, Germany, studied 29 sets of teeth from 16th century skeletons found in the Oaxaca region of Mexico (from a cemetery with known links to the 1545 pandemic); these tests concluded that cocoliztli was most likely an extreme and rare form of the salmonella bacterium, which caused paratyphoid fever. These pandemics coincided with some of the most extreme droughts ever recorded in North America, which exacerbates the spread and symptoms of this disease, and the symptoms described in historical texts give further credence to the claim that cocoliztli was caused by salmonella.

For most of the world, throughout most of human history, the average life expectancy from birth was around 24. This figure fluctuated greatly depending on the time or region, and was higher than 24 in most individual years, but factors such as pandemics, famines, and conflicts caused regular spikes in mortality and reduced life expectancy. Child mortality The most significant difference between historical mortality rates and modern figures is that child and infant mortality was so high in pre-industrial times; before the introduction of vaccination, water treatment, and other medical knowledge or technologies, women would have around seven children throughout their lifetime, but around half of these would not make it to adulthood. Accurate, historical figures for infant mortality are difficult to ascertain, as it was so prevalent, it took place in the home, and was rarely recorded in censuses; however, figures from this source suggest that the rate was around 300 deaths per 1,000 live births in some years, meaning that almost one in three infants did not make it to their first birthday in certain periods. For those who survived to adolescence, they could expect to live into their forties or fifties on average. Modern figures It was not until the eradication of plague and improvements in housing and infrastructure in recent centuries where life expectancy began to rise in some parts of Europe, before industrialization and medical advances led to the onset of the demographic transition across the world. Today, global life expectancy from birth is roughly three times higher than in pre-industrial times, at almost 73 years. It is higher still in more demographically and economically developed countries; life expectancy is over 82 years in the three European countries shown, and over 84 in Japan. For the least developed countries, mostly found in Sub-Saharan Africa, life expectancy from birth can be as low as 53 years.

In each decade between 1629 and 1830 in London, smallpox was responsible for a significant portion of all deaths; having a share of nine or ten percent in the second half of the eighteenth century. After this peak, the discovery of vaccination by Edward Jenner in 1796 helped to bring this share of deaths back down to a similar share it had in the mid-1600s. The lack of statistical data from these early decades makes it difficult to explain why smallpox's share of total deaths was lower than the decades where inoculation (known as "variolation" when referring to smallpox) was in practice throughout Britain, after its introduction in the 1720s; this is possibly due to the impact of other infectious diseases from the time (for example, the bubonic plague was present in England until the late seventeenth century) or possibly the misreporting of data.

The Franco-Prussian War was a ten-month-long conflict between France and the states of Northern Germany; the war itself was pivotal in creating a united German state, and establishing Germany as one of Europe's most powerful nations. One of the forgotten outcomes of the war was the last smallpox outbreak to reach pandemic levels across Europe; this pandemic would be responsible for an estimated 500,000 deaths overall, and led to much stricter vaccination laws being implemented across much of the continent. Prussian soldiers protected, but not civilians In the years leading up to the war, the smallpox death rate among the Prussian civilian population was already more than 33 times higher than it was in the army. This was due to the army's introduction of mandatory revaccination upon enlistment in 1834, and, because of this policy, the Prussian army suffered just 238* smallpox deaths during the war. In contrast to this, vaccination was encouraged but not compulsory in the rest of Prussia, which meant that a large portion of the population were vulnerable to the disease. Vaccination in France and the outbreak of the pandemic While France had been the last European country to embrace widespread inoculation in the eighteenth century, vaccination was not met with as much suspicion. In particular, religious leaders generally welcomed vaccination and promoted its use among all children in the country, however the unstable political leadership and administrations of the mid-1800s failed to make the practice mandatory. The Napoleonic regime had introduced compulsory vaccination in the army in the 1810s, but this was not enforced in the decades that followed his defeat, and vaccination coverage among new recruits in the French army had fallen below fifty percent in 1869. Between 1862 and 1872 (but not including the war), smallpox was responsible for almost one fifth of all deaths in the French army; increased mobilization allowed the disease to spread even further and faster during the Franco-Prussian War.

From limited records during the war, we can see a stark contrast in the impact of smallpox on both sides, with individual garrisons of French soldiers recording more smallpox deaths than the entire German Army*. While just under 2,000 deaths were recorded among French prisoners of war, the impact of these outbreaks on nearby Prussian civilians was devastating; killing over 2.4 thousand and 2.6 thousand per million people in 1871 and 1872 respectively. When the epidemic reached other countries it had a severe effect on the smallpox death rates, particularly in countries without compulsory vaccination such as Belgium and the Netherlands. This pandemic caused countries such as England and Sweden to introduce enforced vaccination, where parents would be punished for not vaccinating their children, while Germany and the Netherlands brought in compulsory vaccination. Despite the pandemic taking thousands of lives in France (the figures given by the French minister of war were seen as being well below the actual number and were quickly disregarded by most scientists), the French government was slow to react with any meaningful legislation; compulsory revaccination for all new army recruits was introduced in 1888, while vaccination was not made compulsory for all civilians until 1902.

The region of present-day China has historically been the most populous region in the world; however, its population development has fluctuated throughout history. In 2022, China was overtaken as the most populous country in the world, and current projections suggest its population is heading for a rapid decline in the coming decades. Transitions of power lead to mortality The source suggests that conflict, and the diseases brought with it, were the major obstacles to population growth throughout most of the Common Era, particularly during transitions of power between various dynasties and rulers. It estimates that the total population fell by approximately 30 million people during the 14th century due to the impact of Mongol invasions, which inflicted heavy losses on the northern population through conflict, enslavement, food instability, and the introduction of bubonic plague. Between 1850 and 1870, the total population fell once more, by more than 50 million people, through further conflict, famine and disease; the most notable of these was the Taiping Rebellion, although the Miao an Panthay Rebellions, and the Dungan Revolt, also had large death tolls. The third plague pandemic also originated in Yunnan in 1855, which killed approximately two million people in China. 20th and 21st centuries There were additional conflicts at the turn of the 20th century, which had significant geopolitical consequences for China, but did not result in the same high levels of mortality seen previously. It was not until the overlapping Chinese Civil War (1927-1949) and Second World War (1937-1945) where the death tolls reached approximately 10 and 20 million respectively. Additionally, as China attempted to industrialize during the Great Leap Forward (1958-1962), economic and agricultural mismanagement resulted in the deaths of tens of millions (possibly as many as 55 million) in less than four years, during the Great Chinese Famine. This mortality is not observable on the given dataset, due to the rapidity of China's demographic transition over the entire period; this saw improvements in healthcare, sanitation, and infrastructure result in sweeping changes across the population. The early 2020s marked some significant milestones in China's demographics, where it was overtaken by India as the world's most populous country, and its population also went into decline. Current projections suggest that China is heading for a "demographic disaster", as its rapidly aging population is placing significant burdens on China's economy, government, and society. In stark contrast to the restrictive "one-child policy" of the past, the government has introduced a series of pro-fertility incentives for couples to have larger families, although the impact of these policies are yet to materialize. If these current projections come true, then China's population may be around half its current size by the end of the century.

It is only in the past two centuries where demographics and the development of human populations has emerged as a subject in its own right, as industrialization and improvements in medicine gave way to exponential growth of the world's population. There are very few known demographic studies conducted before the 1800s, which means that modern scholars have had to use a variety of documents from centuries gone by, along with archeological and anthropological studies, to try and gain a better understanding of the world's demographic development. Genealogical records One such method is the study of genealogical records from the past; luckily, there are many genealogies relating to European families that date back as far as medieval times. Unfortunately, however, all of these studies relate to families in the upper and elite classes; this is not entirely representative of the overall population as these families had a much higher standard of living and were less susceptible to famine or malnutrition than the average person (although elites were more likely to die during times of war). Nonetheless, there is much to be learned from this data. Impact of the Black Death In the centuries between 1200 and 1745, English male aristocrats who made it to their 21st birthday were generally expected to live to an age between 62 and 72 years old. The only century where life expectancy among this group was much lower was in the 1300s, where the Black Death caused life expectancy among adult English noblemen to drop to just 45 years. Experts assume that the pre-plague population of England was somewhere between four and seven million people in the thirteenth century, and just two million in the fourteenth century, meaning that Britain lost at least half of its population due to the plague. Although the plague only peaked in England for approximately eighteen months, between 1348 and 1350, it devastated the entire population, and further outbreaks in the following decades caused life expectancy in the decade to drop further. The bubonic plague did return to England sporadically until the mid-seventeenth century, although life expectancy among English male aristocrats rose again in the centuries following the worst outbreak, and even peaked at more than 71 years in the first half of the sixteenth century.

Throughout the Common Era, Japan's population saw relatively steady growth between each century. Failed invasions and distance from Asia's mainland meant that Japan was unaffected by many pandemics, primarily bubonic plague, therefore its development was not drastically impeded in the same way as areas such as China or Europe. Additionally, religious practices meant that hygiene was prioritized much more in Japan than in other regions, and dietary customs saw lower rates of meat consumption and regular boiling of water in meals or tea; both of these factors contributed to lower rates of infection for many parasitic or water-borne diseases. Fewer international conflicts and domestic stability also saw lower mortality in this regard, and Japan was an considered an outlier by Asian standards, as some shifting trends associated with the demographic transition (such as lower child mortality and fertility) began taking place in the 17th century; much earlier time than anywhere else in the world. Yet the most significant changes came in the 20th century, as Japan's advanced healthcare and sanitation systems saw drastic reductions in mortality. Challenges Japan's isolation meant that, when pandemics did arrive, the population had less protection and viruses could have higher mortality rates; smallpox has been cited as the deadliest of these pandemics, although increased international contact in the late 19th century brought new viruses, and population growth slowed. Earlier isolation also meant that crop failure or food shortages could leave large sections of the population vulnerable, and, as mentioned, the Japanese diet contained relatively little meat, therefore there was a higher reliance on crops and vegetables. It is believed that the shortage of arable land and the acidity of the soil due to volcanic activity meant that agriculture was more challenging in Japan than on the Asian mainland. For most of history, paddy fields were the most efficient source of food production in Japan, but the challenging nature of this form of agriculture and changes in employment trends gradually led to an increased reliance in imported crops. Post-Sakoku Japan Distance from the Asian mainland was not the only reason for Japan's isolation; from 1603 to 1853, under the Tokugawa shogunate, international trade was restricted, migration abroad was forbidden, and most foreign interaction was centered around Nagasaki. American neo-imperialism then forced Japan to open trade with the west, and Japan became an imperial power by the early-1900s. Japanese expansion began with a series of military victories against China and Russia at the turn of the century, and the annexation of Taiwan, Korea, and Manchuria by the 1930s, before things escalated further during its invasion of China and the Second World War. Despite its involvement in so many wars, the majority of conflicts involving Japan were overseas, therefore civilian casualties were much lower than those suffered by other Asian countries during this time. After Japan's defeat in 1945, its imperial ambitions were abandoned, it developed strong economic ties with the West, and had the fastest economic growth of any industrial country in the post-WWII period. Today, Japan is one of the most demographically advanced countries in the world, with the highest life expectancy in most years. However, its population has been in a steady decline for over a decade, and low fertility and an over-aged society are considered some of the biggest challenges to Japanese society today.