OUR WORLD IN DATA – COVID-19 has brought the reality of pandemics to the forefront of public consciousness. But pandemics have afflicted humanity for millennia. Time and again, people faced outbreaks of diseases – including influenza, cholera, bubonic plague, smallpox, and measles – that spread far and caused death and devastation.
Our ancestors were largely powerless against these diseases and unable to evaluate their true toll on the population. Without good record-keeping of the number of cases and deaths, the impact of outbreaks was underrecognized or even forgotten. The result is that we tend to underestimate the frequency and severity of pandemics in history.
Often, we have records of epidemics occurring in some countries but lack good records from other regions, despite knowing that the geographical impact of the disease would have been very wide.
Additionally, we often lack knowledge about which pathogens caused outbreaks and, thus, if a historical event can be considered a pandemic or if it consisted of parallel outbreaks of different diseases.
To deal with the lack of historical records on total death tolls, modern historians, epidemiologists, and demographic researchers have used various sources and methods to estimate their death tolls – such as using data from death records, tax registers, land use, archaeological records, epidemiological modeling, and more.
In this article, I present the various methods they rely on and visualize the estimated impact of what are now considered the major pandemics in history.
What is a pandemic?
Although there is no universally accepted definition of a pandemic1, diseases called pandemics share several characteristics.
Pandemics generally refer to diseases with a vast geographic range – such as spreading across a continent or multiple continents. In addition, they tend to describe outbreaks that are rapidly growing or expanding in range; highly infectious; affecting a large number of people; and caused by novel pathogens against which there is little or no pre-existing immunity.2
How do researchers estimate the death toll of pandemics?
Researchers have estimated the death tolls of pandemics in different ways, depending on the data available.
Some death tolls have been estimated by looking at excess deaths: researchers estimate the additional number of deaths that occurred during a pandemic compared to the expected number of deaths in a typical year. This can be helpful to understand the pandemic’s overall impact, even if records from death certificates are unavailable.
For some pandemics, death tolls are estimated from the net population reduction, where researchers calculate the difference in population size before and after the pandemic. This is often used for severe events – such as the Columbian Exchange – where a significant fraction of the population died.
Some death tolls have been estimated through epidemiological modeling – based on knowledge of the transmission of the disease and its geographical spread, its fatality rate (the share of people affected who die from it), access to treatment, and other types of data.
Finally, some death tolls have been calculated only using recorded deaths (also referred to as ‘confirmed deaths’). This is the number of deaths officially reported with the disease as their cause of death. This method may vastly underestimate the number of deaths caused by the pandemic, as comprehensive historical records are lacking. Even today, cause-of-death registration is lacking in many parts of the world, which is one reason why the number of confirmed deaths from COVID-19 is much lower than the total death toll from the pandemic.
A timeline of historical pandemics
I have brought together estimates of death tolls from different pandemics in history for this article, which we have visualized in a timeline below.
The size of each circle represents one pandemic’s estimated death toll. Pandemics without a known death toll are depicted with triangles.