Do pandemics happen every 100 years? No, that's not true: A post making the claim cherry-picks disease outbreaks, excluding obvious examples. It also misrepresents basic facts about the pandemics it does list, including the years in which the outbreaks began.
The claim appeared in an Instagram post (archived here) published January 20, 2023. It included a short slideshow of images with a caption that opened:
Pandamics happens every 100 years
This is what the post looked like on Instagram at the time of writing:
(Source: Instagram screenshot taken on Mon Feb 6 15:32:54 2023 UTC)
The post identified four purported pandemics. They were listed as follows:
The Great Plague of Marseile (1720)
Cholera outbreak (1820)
Spanish flu (1920)
Coronavirus outbreak (2020)
There are at least two problems with the list. One, it misrepresents some basic facts about the pandemics, including the years in which the outbreaks began.
The Spanish flu began in 1918, not 1920, and the cholera outbreak started in 1817, not 1820, according to the Robert Wood Johnson Foundation, the largest health foundation in the United States. The first cases of COVID-19 appeared in December 2019, not 2020, according to the U.S. Centers for Disease Control and Prevention (CDC).
Also, what the post identifies as the "Great Plague of Marseile" is arguably not a pandemic. The CDC defines a pandemic as "an epidemic that has spread over several countries or continents, usually affecting a large number of people." Although the plague claimed the lives of nearly half the population of Marseille, the outbreak was largely contained within France.
The second problem with the list is that it excludes obvious examples of other pandemics. For example, it does not include "The Plague," also known as the Black Death, which struck in the middle of the 14th (1347) century and is estimated to have killed as many as 200 million people. The list also does not include the 1957-58 influenza pandemic, caused by the H2N2 virus, nor does it include the HIV/AIDS pandemic, which is ongoing, with the first cases noted in 1981.
