Deadliest Pandemics and Epidemics in History

Data

Note: What Makes a Pandemic Deadly?

Epidemic, Pandemic, Endemic: What Is the Difference?

• Epidemic - a disease spreading through a community or region at a rate well above what is normally expected. A city-wide measles outbreak counts.
• Pandemic - an epidemic that has crossed borders and continents and is spreading person to person in many countries at once. The World Health Organization declares a pandemic when a new pathogen is spreading globally, not when it is especially lethal.
• Endemic - a disease that is always present in a place at a steady, predictable baseline. Malaria is endemic to much of sub-Saharan Africa; dengue is endemic to parts of Southeast Asia.
• The word pandemic comes from the Greek pan (all) and demos (people). Because the definition is about reach rather than deadliness, the 2009 H1N1 "swine flu" was a pandemic even though it was mild, while Ebola, which can kill half the people it infects, is not one: it is too hard to catch to circle the globe.

How Historians Estimate Death Tolls

Nobody was standing at the door in 1346 counting corpses. Every number on this page is an estimate reconstructed from evidence, which is why almost every entry appears as a range rather than a single figure.

• Contemporary records. Parish burial registers, tax rolls, monastery chronicles and London's weekly "Bills of Mortality" (published from 1592) let historians compare the dead of a plague year with a normal year.
• Excess mortality. The modern gold standard: count all deaths during an outbreak and subtract the deaths that would normally have been expected. This is how researchers concluded that COVID-19's true toll was several times the confirmed figure.
• Ancient DNA and archaeology. Genetic material pulled from the teeth of plague victims confirmed in 2011 that Yersinia pestis caused the Black Death, and a 2018 study identified Salmonella enterica DNA in 16th-century Mexican cocoliztli victims. Bones can settle arguments that documents cannot.
• Why the ranges are so wide. Estimates for the Black Death have ranged from roughly 25 million to over 200 million, depending on whether historians count only Europe or the whole known world, and whether famine and war deaths are blamed on the outbreak. This page ranks events by the midpoint of the published range, and the data table above shows the full range for each one.

The Three Plague Pandemics

Bubonic plague is caused by the bacterium Yersinia pestis, carried by fleas that live on rats and other rodents. It has produced three of history's worst outbreaks.

• Plague of Justinian (541-549) - named for the Byzantine emperor who fell ill with it, and the first recorded plague pandemic, hitting the Mediterranean world shortly after Rome's western half had already collapsed.
• Black Death (1346-1353) - the pandemic that killed somewhere between a third and a half of Europe's population, emptied villages, and helped reshape wages and land ownership in ways that fed into the Renaissance.
• Third plague pandemic (1855-1960) - began in Yunnan, China, and rode steamships and railways around the world. It reached Hong Kong in 1894, where Alexandre Yersin identified the bacterium that now bears his name, then devastated India.
• Plague never actually left. It is endemic in the western United States, Madagascar, the Democratic Republic of the Congo and Peru, causing a few hundred cases a year. Treated early with antibiotics, it is curable.

Influenza, the Recurring Pandemic

• The 1918 "Spanish flu" earned its nickname for a political reason: Spain stayed neutral in World War I and reported the outbreak openly, while combatant countries censored news to protect morale. The first recorded wave actually struck Kansas in early 1918.
• It was an influenza A/H1N1 virus, and it killed young adults aged 20 to 40 in unusual numbers, producing a W-shaped mortality curve instead of the J-shaped one that flu normally causes. It killed more people in about six months than the First World War killed in four years.
• The 1957-1958 Asian flu (H2N2) and the 1968 Hong Kong flu (H3N2) each killed somewhere between one and four million people, showing that pandemic influenza returned whenever the virus shuffled its genes.

The Only Disease We Have Ever Erased

• Smallpox, caused by the variola virus, killed an estimated 300 million people in the 20th century alone, more than all the century's wars combined.
• The 735-737 Japanese epidemic killed roughly a third of Japan's population. After 1492, smallpox and other imported infections caused the largest die-off in the Americas, including the 1520 Mexico epidemic that killed millions as the Aztec empire was already in crisis.
• Edward Jenner's 1796 experiment with cowpox material started vaccination, and coordinated vaccination campaigns led the World Health Organization to declare smallpox eradicated in 1980, the only human disease ever eliminated.
• Cocoliztli, the mystery disease that killed millions in 16th-century Mexico (5 to 15 million in 1545-1548 alone), is still debated. It coincided with severe drought and famine, and a 2018 ancient DNA study pointed to paratyphoid fever, though researchers disagree.

Modern Pandemics and What Changed

• HIV/AIDS was first reported in 1981 and its virus identified in 1983. It has killed around 45 million people, but antiretroviral therapy, available since 1996, turned a fatal diagnosis into a manageable chronic condition. Around 40 million people live with HIV today.
• COVID-19 was declared a pandemic on 11 March 2020. Roughly 7 million deaths were confirmed, while studies of excess mortality suggest the real toll was 20 million or more. Vaccines reached arms in under a year, the fastest vaccine development in history (the previous record, for mumps, took about four years).
• Cholera has caused seven pandemics since 1817. John Snow's 1854 investigation of the Broad Street water pump in London created modern epidemiology, and cholera still infects 1.3 to 4 million people and kills tens of thousands every year.
• Typhus spreads through body lice, which is why it thrives in armies, refugee camps and prisons. The 1918-1922 Russian epidemic killed 2 to 3 million people during civil war and famine.

Total Deaths vs Share of Population

Ranked by total deaths, modern events dominate this list simply because there are far more people to infect: world population grew from about 1.9 billion in 1918 to over 8 billion today. Ranked by the share of the population lost, the older outbreaks win by a mile. The Black Death killed an estimated 30 to 60 percent of Europe and the Plague of Justinian 25 to 60 percent, while the 1918 flu killed somewhere between 1 and 5 percent of the world, and COVID-19 about 0.4 percent. Both measures are honest; they simply answer different questions.

Fun Facts

• The word quarantine comes from the Venetian quaranta giorni, the 40 days ships had to wait in the lagoon under a 1377 plague rule before passengers could come ashore.
• Serial interval and R0, the average number of people one case infects, decide how big an outbreak can get. The share of a population that must be immune to stop a disease is roughly 1 minus 1/R0: for measles, with an R0 of 12 to 18, that means 92 to 95 percent.
• The 1918 flu killed more Americans than combat did in World War I (about 675,000 deaths versus roughly 53,000 in battle).
• Plague is older than history: DNA from a 5,000-year-old skeleton pushed its origin back to the Bronze Age, thousands of years before the Black Death.
• Lethality alone is not destiny. A pathogen that kills 30 percent of the people it infects but spreads poorly is far less destructive to a population than one that kills 2 percent of everyone on Earth.
• The 1918 influenza killed more people in about 24 weeks than HIV/AIDS has killed in more than 40 years.
• Wastewater surveillance, used worldwide during the COVID-19 pandemic, is a direct descendant of the 1854 London cholera investigations that traced an outbreak to a single water pump.