Plague of Justinian in the context of "Volcanic winter of 536"

This is a list of the largest known epidemics and pandemics caused by an infectious disease in humans. Widespread non-communicable diseases such as cardiovascular disease and cancer are not included. An epidemic is the rapid spread of disease to a large number of people in a given population within a short period of time; in meningococcal infections, an attack rate in excess of 15 cases per 100,000 people for two consecutive weeks is considered an epidemic. Due to the long time spans, the first plague pandemic (6th century – 8th century) and the second plague pandemic (14th century – early 19th century) are shown by individual outbreaks, such as the Plague of Justinian (first pandemic) and the Black Death (second pandemic).

Infectious diseases with high prevalence are listed separately (sometimes in addition to their epidemics), such as malaria, which may have killed 50–60 million people.

Yersinia pestis (Y. pestis; formerly Pasteurella pestis) is a gram-negative, non-motile, coccobacillus bacterium without spores. It is related to pathogens Yersinia enterocolitica, and Yersinia pseudotuberculosis, from which it evolved. Yersinia pestis is responsible for the disease plague, which caused the Plague of Justinian and the Black Death, one of the deadliest pandemics in recorded history. Plague takes three main forms: pneumonic, septicemic, and bubonic. Y. pestis is a facultative anaerobic parasitic bacterium that can infect humans primarily via its host the Oriental rat flea (Xenopsylla cheopis), but also through aerosols and airborne droplets for its pneumonic form. As a parasite of its host, the rat flea, which is also a parasite of rats, Y. pestis is a hyperparasite.

Y. pestis was discovered in 1894 by Alexandre Yersin, a Swiss/French physician and bacteriologist from the Pasteur Institute, during an epidemic of the plague in Hong Kong. Yersin was a member of the Pasteur school of thought. Kitasato Shibasaburō, a Japanese bacteriologist who practised Koch's methodology, was also engaged at the time in finding the causative agent of the plague. However, Yersin actually linked plague with a bacillus, initially named Pasteurella pestis; it was renamed Yersinia pestis in 1944.

The second plague pandemic was a major series of epidemics of plague that started with the Black Death, which reached medieval Europe in 1346 and killed up to half of the population of Eurasia in the next four years. It followed the first plague pandemic that began in the 6th century with the Plague of Justinian, which ended in the 8th century. Although the plague died out in most places after 1353, it became endemic and recurred regularly. A series of major epidemics occurred in the late 17th century, and the disease recurred in some places until the late 18th century or the early 19th century. After this, a new strain of the bacterium gave rise to the third plague pandemic, which started in Asia around the mid-19th century.

By the early 19th century, the threat of plague had diminished, though it was quickly replaced by the spread of another deadly infectious disease in the first cholera pandemic, beginning in 1817, the first of several cholera pandemics to sweep through Asia and Europe during the 19th and 20th centuries. One of the last epidemics to strike the Balkans during the second plague pandemic was Caragea's plague, between 1813 and 1814. The Swiss explorer Johann Ludwig Burckhardt witnessed the plague epidemics that ravaged Hejaz and Egypt between 1812 and 1816. He wrote: "In five or six days after my arrival [in Yanbu] the mortality increased; forty or fifty persons died in a day, which, in a population of five or six thousand, was a terrible mortality."Malta had its second most severe plague epidemic in the epidemic of 1813–1814, which killed around 4,500 people.

Early Slavs began mass migrating to Southeastern Europe between the first half of the 6th and 7th century in the Early Middle Ages. The rapid demographic spread of the Slavs was followed by a population exchange, mixing and language shift to and from Slavic.

The settlement was facilitated by the substantial decrease of the Southeastern European population during the Plague of Justinian. Another reason was the Late Antique Little Ice Age from 536 to around 660 CE and the series of wars between the Sasanian Empire and the steppe nomads against the Eastern Roman Empire. After the arrival of the Pannonian Avars in the mid-6th century, they continued to conduct incursions into Roman territory, often independently of Avar's influence. After the failed siege of Constantinople in the summer of 626, and successful revolt against the Avars, they remained in the wider Southeast Europe area after they had settled the Byzantine provinces south of the Sava and Danube rivers, from the Adriatic Sea to the Aegean and Black Sea.

The settlement of Great Britain by Germanic peoples from continental Europe led to the development of an Anglo-Saxon cultural identity and a shared Germanic language—Old English—whose closest known relative is Old Frisian, spoken on the other side of the North Sea. The first Germanic speakers to settle in Britain permanently are likely to have been soldiers recruited by the Roman administration in the 4th century AD, or even earlier. In the early 5th century, during the end of Roman rule in Britain and the breakdown of the Roman economy, larger numbers arrived, and their impact upon local culture and politics increased.

There is ongoing debate about the scale, timing and nature of the Anglo-Saxon settlements and also about what happened to the existing populations of the regions where the migrants settled. The available evidence includes a small number of medieval texts which emphasize Saxon settlement and violence in the 5th century but do not give many clear or reliable details. Linguistic, archaeological and genetic information have played an increasing role in attempts to better understand what happened. The British Celtic and Latin languages spoken in Britain before Germanic speakers migrated there had very little impact on Old English vocabulary. According to many scholars, this suggests that a large number of Germanic speakers became important relatively suddenly. On the basis of such evidence it has even been argued that large parts of what is now England were clear of prior inhabitants, perhaps due to mass deaths from the Plague of Justinian. However, a contrasting view that gained support in the late 20th century suggests that the migration involved relatively few individuals, possibly centred on a warrior elite, who popularized a non-Roman identity after the downfall of Roman institutions. This hypothesis suggests a large-scale acculturation of natives to the incomers' language and material culture. In support of this, archaeologists have found that, despite evidence of violent disruption, settlement patterns and land use show many continuities with the Romano-British past, despite profound changes in material culture.

The population of the Byzantine Empire encompassed all ethnic and tribal groups living there, mainly Byzantine Greeks, but also Albanians, Armenians, Assyrians, Bulgarians, Goths, Kartvelians, Latini, Levantine Arabs, Serbs and Croats, Thracians, Tzans, Vlachs and other groups. It fluctuated throughout the state's millennial history. The reign of the Emperor Justinian I in the mid-sixth century was the high point of the empire's expansion; however, the arrival of plague in 541 AD and its subsequent recurrences caused a severe depletion of the population. After the reign of Emperor Heraclius (r. 610–641 AD) and the loss of the empire's overseas territories, Byzantium was limited to the Balkans and Anatolia. When the empire began to recover after a series of conflicts in the 8th century and its territories stabilized, its population began to recover. By the end of the 8th century the population of the empire was around 7,000,000, a figure that climbed to over 12,000,000 people by 1025 AD. The numbers began falling steadily to 9,000,000 people at 1204 AD and even lower to 5,000,000 people at 1282 AD with the arrival of the Turks.

The first plague pandemic was the first historically recorded Old World pandemic of plague, the contagious disease caused by the bacterium Yersinia pestis. Also called the early medieval pandemic, it began with the Plague of Justinian in 541 and continued until 750 or 767. At least fifteen to eighteen major waves of plague following the Justinianic plague have been identified from historical records. The pandemic affected the Mediterranean Basin most severely and most frequently, but also infected the Near East and Northern Europe, and potentially East Asia as well. The Roman emperor Justinian I's name is sometimes applied to the whole series of plague epidemics in late antiquity.

The pandemic is best known from its first and last outbreaks: the Justinianic Plague of 541–549, described by the contemporary Roman historian Procopius, and the late 8th century plague of Naples described by Neapolitan historian John the Deacon in the following century (distinct from the Naples Plague of the 17th century, during the second plague pandemic). Other accounts from contemporaries of the pandemic are included in the texts of Evagrius Scholasticus, John of Ephesus, Gregory of Tours, Paul the Deacon, and Theophanes the Confessor; most seem to have believed plague was a divine punishment for human misdeeds.

The third plague pandemic was a major bubonic plague pandemic that began in Yunnan, China, in 1855. This episode of bubonic plague spread to all inhabited continents, and ultimately led to more than 12 million deaths in India and China (and perhaps over 15 million worldwide), and at least 10 million Indians were killed in British Raj India alone, making it one of the deadliest pandemics in history. According to the World Health Organization, the pandemic was considered active until 1960, when worldwide casualties dropped to 200 per year. Plague deaths have continued at a lower level for every year since.

The name refers to the third of at least three known major plague pandemics. The first began with the Plague of Justinian, which ravaged the Byzantine Empire and surrounding areas in 541 and 542; the pandemic persisted in successive waves until the middle of the 8th century. The second began with the Black Death, which killed at least one third of Europe's population in a series of expanding waves of infection from 1346 to 1353; this pandemic recurred regularly until the 19th century.