Fact Check

Have Plagues Repeated Exactly Every 100 Years?

The predictive value of cherry-picked data is generally low to insignificant.

Published April 7, 2020

 (La Franceschina, ca 1474)
Image Via La Franceschina, ca 1474
A meme documents that plagues occur every 100 years.
What's True

Pandemics of some type were extant during all four of the years referenced in the meme.

What's False

However, the meme cherry-picks a small amount of data and isn't particularly useful in any predictive sense.

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A common phenomenon that occurs during crises (e.g., natural disasters, political upheaval, terrorist attacks) is attempts by people to assert a sense of certainty and control over events by finding patterns in them, thereby suggesting that whatever is occurring — however chaotic or frightening it may be — is nonetheless natural, predictable, or manageable. So it was with the following meme, which was circulated during the COVID-19 coronavirus disease pandemic in April 2020:

plagues 100 years

Although this meme couldn't fairly be classified as "false" (since pandemics of some type were indeed extant during all four of the years it references), it isn't particularly useful in any informative or predictive sense.

It's an example of the common technique of creating the impression of a regular pattern by cherry-picking a small amount of (not necessarily relevant) data, while completely ignoring a much larger body of related data that doesn't fit the desired pattern. Specifically:

  • The meme omitted every century ending in 20 prior to 1720, even though the occurrence of pandemics has been charted since long before the early 18th century.
  • The meme comprised only four entries, not a particularly impressive or convincing amount of data (especially in light of the above point).
  • The meme made no allowance for all the pandemics that spanned years other than those ending in 20 (which might suggest a completely different pattern, or none at all).
  • The listed pandemics shared no substantive commonality (such as underlying disease, geographic area, duration, extent) that was indicative of any meaningful pattern.

Nonetheless, for those who want to know how "true" the meme is, we offer the following summary of what its entries refer to:

  • Beginning in 1720, an outbreak of bubonic plague in Marseille, France (known as The Great Plague of Marseille) killed an estimated 100,000 people in that city and surrounding provinces and towns. However, that particular outbreak was far from the first, last, or most severe instance of bubonic plague in history. In particular, the Black Death — a bubonic plague epidemic that hit Eurasia in the 14th century — is estimated to have killed 30% to 60% of Europe's human population (up to 50 million people), primarily from 1347 to 1351.
  • The first of several cholera pandemics recorded in modern history spread from India to Southeast Asia, the Middle East, Europe, and Eastern Africa in the early 19th century. However, that pandemic did not begin in 1820. It persisted from 1817 until 1824, and six more cholera pandemics were charted over the following 150 years.
  • The so-called "Spanish flu" or 1918 flu pandemic (influenza caused by an H1N1 virus) spread in the early 20th century, killing upwards of 50 million people worldwide. (Despite the name, most modern scholarship suggests the pandemic did not actually originate in Spain.) Once again, although that pandemic did encompass the year 1920, it began much earlier, continuing roughly from January 1918 to December 1920.
  • The COVID-19 coronavirus disease outbreak that made this image of interest in 2020 was initially reported at the end of 2019, but it was not officially characterized as a pandemic by the World Health Organization until March 2020.

David Mikkelson founded the site now known as snopes.com back in 1994.

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