The virtual eradication of the plague. An unsolved riddle of health progress

Boris V. Schmid. Show Author details

Infectious diseases have long been one of the leading causes of death for humankind. Luckily, their impact has declined during the 19th and 20th century, as detailed by others on this blog (see here or here). Even the Spanish Flu, the AIDS pandemic, and the current COVID-19 pandemic do not reverse that trend.

The Eradication of Disease

If we move further back in time, we find that two of the most lethal diseases in history, namely smallpox and the plague, have been with us for thousands of years. We know this because historic sources talk about the distinct symptoms of these diseases and because scientists have recovered genetic fragments of these diseases from human remains, as plague DNA is frequently found in ancient human remains. The map below shows the location and age of the oldest plague samples, based on the excellent work of various ancient DNA groups. 

However, it is very hard to reconstruct the demographic impact these diseases had on human populations that far in the past. It is easier to reconstruct the impact of diseases in more recent times. We have sources like death registers that hold detailed information on the death toll of smallpox and the plague relative to other causes. Smallpox, for example, caused an astonishing 8-10% of all deaths in 18th century London. As you can see in the figure below, vaccination completely changed that, but it took almost a hundred years of increasing uptake and efficiency to do so (Krylova and Earn, 2020). 

By 1980, smallpox was declared to be eradicated from the world. The eradication of smallpox and the success of vaccination are often hailed as one of the great triumphs of modern medicine, and rightly so. Moreover, the prospect of eradicating diseases has an innate appeal to many people, including fundraisers and politicians, as it neatly solves and closes off a risk. However, despite our attempts to eliminate several other diseases (see here), we have only had one further success, with the eradication of rinderpest in 2011.

Plague has never been eradicated, and the disease is still widespread in wildlife rodents as the map below shows. Part of my work focuses on improving this map, as it over-represents the area where plague exists by a fairly large fraction. Nevertheless, cases in humans have dropped dramatically and are now limited to about 100-200 people per year, as data from WHO show (here). Barring a few hot spots, plague has virtually disappeared from humanity. This is a remarkable success story.

Yet the disappearance of plague is not celebrated in the way that the eradication of smallpox is. Perhaps that is because plague has not been conquered completely, like smallpox, or because we cannot point to a singular action that we took that caused the disappearance of plague, like we can with the discovery and rollout of the smallpox vaccine.

The Decline of Mortality 

How deadly were the medieval and early-modern plague outbreaks? The last wave of plague outbreaks recorded in Europe, such as the Great Plague of London in the 17th century, had a death rate of ≈20% of the population (Earn et al., 2020). I would say that a death toll of one-fifth of the population is fairly typical for the major plague outbreaks that struck towns every 20-40 years during the 14th-18th century in Europe (Roosen and Curtis, 2019; Curtis, 2016). 

But fast forward just two centuries to the Third Plague Pandemic of the late 19th and early 20th century, and most of the world reported a total mortality of less than 0.15% of the population (see table below).  For this blog post I took the mortality numbers from various sources at face value, but a more thorough study would look deeper into reporting biases. 

Even in the hardest-struck regions of the world (India, Southern China, and Taiwan), the death toll stopped at 4-8% of the population: a distressingly high number of people – COVID today is estimated at just 0.25% (see here). But compared to the earlier plague outbreaks, the butchers’ bill of the 20th century was far milder than the plague pandemics of old.

Today’s situation is even better, and countries like Kazakhstan, with  about 3 million people living in wildlife plague foci (39% of the country’s territory), suffered on average 1 case per year in the last decades (Sagiev, Z.A. et al., 2019). Even the sizeable urban plague outbreak in Madagascar in 2017 had only 209 casualties, a small fraction of the 1.5 million people living in the two largest cities affected (Spiegel, 2019).

How did we virtually eradicate the plague?

How did the relative death toll of plague fall from up to 20% in 17th century Europe to close to 0% today? 

First, consider that plague is a disease that spreads predominantly through flea bites. Therefore, the best way to understand why the mortality fell is to look for societal changes that reduced our exposure to flea bites – from rat fleas and potentially also from human fleas (Dean et al., 2018). For instance, we have improved the building quality of our homes, stopped living with livestock, and have been heating our rooms to higher temperatures and lower humidity, all of which negatively affect the number of fleas living in our homes. Also, we changed our bedding material from straw to other materials and have improved our overall hygiene and that of our environment. We wash ourselves more regularly and store our food in fridges and cans where they are no longer accessible to rats. There is a myriad of potential explanations! 

But we are not looking for relatively recent changes. The health improvements in Europe of the late 19th and early 20th century are impressive, but plague had already disappeared from Europe by then. Therefore, we are looking for progress made during the 17th and early-18th century in Europe. Similar transitions from high to low plague mortality happened in the Middle East, Asia and Africa, which should be studied in the context of their health improvements. 

The lack of a clear answer in this paragraph should be a dead giveaway to readers that the decline of plague is still very much an open research question, and perhaps it is one that you can help solve.

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• Curtis, D.R. (2016) ‘Was plague an exclusively urban phenomenon? Plague mortality in the seventeenth-century Low Countries’, The Journal of interdisciplinary history, 47(2), 139-170.
• Dean, K.R. et al. (2018) ‘Human ectoparasites and the spread of plague in Europe during the Second Pandemic’, Proceedings of the National Academy of Sciences of the United States of America, 115(6): 181695.
• Earn, D.J.D. et al. (2020) ‘Acceleration of plague outbreaks in the second pandemic’, Proceedings of the National Academy of Sciences of the United States of America, 117(44), 27703-27711.
• Krylova, O. and Earn, D.J.D. (2020) ‘Patterns of smallpox mortality in London, England, over three centuries’, PLoS biology, 18(12), 1-27.
• Roosen, J. and Curtis, D.R. (2019) ‘The “light touch” of the Black Death in the Southern Netherlands: an urban trick?’, The Economic history review, 72(1), 32-56. 
• Sagiev, Z.A. et al. (2019) ‘Diseases of human plague in 1974-2003 in Kazakhstan’, Ekoloji, 28(107), 39-48.
• Spiegel, A. (16 April 2019) ‘Lessons learnt from the 2017/2018 plague outbreak in Madagascar’, in Global health: African update. ECCMID 2019.

Remarks about the numbers provided in the table above

I did not try to compensate for under-reporting. Also, large parts of the world where the Third Pandemic also took place are missing (e.g. Russia, large parts of Asia or the middle East), for which I had no data at hand.

The percentage of deaths for Southern China and Taiwan are based on the most-affected provinces  (Yunnan, Guangdong, Hainan, Jiangxi, Fujian, Zhejiang) and Taiwan. As such, their mortality percentages may be inflated compared to regions whose populations were only partially exposed to plague.

The sources used are:

• Moll, A.A. and O’Leary, S.B. (1941) ‘Plague in the Americas: an historical and quasi-epidemiological survey’, Pan American Journal of Public Health. page 461.
• Bramanti, B. et al. (2019) ‘The Third Plague Pandemic in Europe’, Proceedings of the Royal Society B: Biological Sciences, 286(1901). I used the data until 1938
• Neerinckx, S., Bertherat, E. and Leirs, H. (2010) ‘Human plague occurrences in Africa: an overview from 1877 to 2008’, Transactions of the Royal Society of Tropical Medicine and Hygiene, 104(2). I used the data until 1938.
• Tennant, W.S.D. et al. (2020) ‘Climate drivers of plague epidemiology in British India, 1898-1949’, Proceedings. Biological sciences / The Royal Society, 287(1928), p. 20200538. I used the summary number of over 13 million. The paper remarks that after 1930, the deaths to plague are very low.
• Ben-Ari, T. et al. (2012) ‘Identification of Chinese plague foci from long-term epidemiological data’, Proceedings of the National Academy of Sciences of the United States of America, 109(21). I used the data underlying this paper, restricted to 1884-1938 and latitudes lower than 31 degrees.

When cases rather than deaths were reported, I used a case fatality rate of about 60%. Using the Third Pandemic period up to 1938 (to make the total death toll comparable between continents). As denominators, I used population size estimates from around 1913, as provided by the following pages from Wikipedia (link, link, link, link).

Remarks about the map with location and age of the oldest plague samples

The map was made with Natural Earth from coordinates and date estimates published in:

• Rascovan, N. et al. (2019) ‘Emergence and Spread of Basal Lineages of Yersinia pestis during the Neolithic Decline’, Cell, 176(1-2), pp. 295–305.e10. doi:10.1016/j.cell.2018.11.005. 
• Andrades Valtueña, A. et al. (2017) ‘The Stone Age Plague and Its Persistence in Eurasia’, Current biology: CB, 27(23), pp. 3683–3691.e8. doi:10.1016/j.cub.2017.10.025. 
• Swali, P. et al. (2022) ‘Yersinia pestis genomes reveal plague in Britain 4,000 years ago’, bioRxiv. doi:10.1101/2022.01.26.477195. 
• Spyrou, M.A. et al. (2018) ‘Analysis of 3800-year-old Yersinia pestis genomes suggests Bronze Age origin for bubonic plague’, Nature communications, 9(1), p. 2234. doi:10.1038/s41467-018-04550-9. 
• Susat, J. et al. (2021) ‘A 5,000-year-old hunter-gatherer already plagued by Yersinia pestis’, Cell reports, 35(13). doi:10.1016/j.celrep.2021.109278. 
• Rasmussen, S. et al. (2015) ‘Early Divergent Strains of Yersinia pestis in Eurasia 5,000 Years Ago’, Cell, 163(3), pp. 571–582. doi:10.1016/j.cell.2015.10.009.

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