The plagues that
broke the world
Before antibiotics, before germ theory, before microscopes, infectious diseases regularly killed a third of a continent's population in a single event. The Black Death alone killed more humans in four years than the First World War killed in four — and left the survivors living in a fundamentally different civilization. This is what we know, and what we finally understand, about the pandemics that shaped history.
"The condition of the people was pitiable to behold. They sickened by the thousands daily, and died unattended and without help. Many died in the open street, others dying in their houses, made it known by the stench of their rotting bodies. Consecrated churchyards did not suffice for the multitude of corpses, which were heaped by the hundreds in vast trenches, like goods in a ship's hold."Giovanni Boccaccio · Decameron, Preface · Florence, 1353
What it is
Yersinia pestis is a gram-negative bacterium in the family Yersiniaceae. It evolved from Yersinia pseudotuberculosis, a gut pathogen, through a small number of genetic mutations that gave it the ability to colonise fleas, spread to the lymphatic system, and overwhelm the immune response — changes that happened within the last 5,000 to 20,000 years on an evolutionary timescale.
In nature the bacterium circulates silently within populations of wild rodents, causing them little harm. Transmission to humans happens through the bite of infected fleas, through contact with infected animal tissue, or — in the pneumonic form — directly between people through respiratory droplets. All three forms are the same bacterium, in different anatomical locations.
How it was confirmed — ancient DNA
The identification of Y. pestis as the cause of the Black Death was debated for over a century — some historians argued for anthrax, viral hemorrhagic fever, or even an unknown disease. The answer came from ancient DNA (aDNA) extracted from the teeth of burial victims.
By the 1990s and 2000s, aDNA from plague pits in France, Germany, and England's East Smithfield mass burial confirmed the bacterium. The 2022 Nature study went further — DNA from seven individuals at Kara-Djigach and Burana cemeteries in Kyrgyzstan, buried with tombstones dated 1338–1339 and inscribed "pestilence," confirmed those graves hold the direct ancestral strain of the Black Death: the "Big Bang" of Y. pestis diversity that seeded all the major lineages that swept the medieval world.
"The physicians in particular suffered, as they visited the sick most often and yet knew not what it was. No remedy was found that could be used as a specific; for what did good to one did harm to another. Strong and weak constitutions proved equally incapable of resistance, all alike being swept away."Thucydides · History of the Peloponnesian War, Book II · on the Plague of Athens, 430 BCE
Why it still circulates
Yersinia pestis is not a human disease that could be eradicated the way smallpox was. It lives in wild rodent populations — ground squirrels, prairie dogs, and chipmunks in the American Southwest; gerbils and rats across Central Asia and Africa — where it causes little harm and has persisted for millennia. As long as those reservoir populations exist, the bacterium does too. Modern rodent die-offs in plague-endemic zones are often the early warning that transmission to humans may follow.
In the United States, plague cases cluster in New Mexico, Arizona, Colorado, and California — the descendants of a Third Pandemic introduction around 1900 that spread into the Western rodent population and has never been eradicated. Cases are typically traceable to contact with infected animals or their fleas during outdoor activities.
Treatment — what changed everything
Streptomycin, the first effective treatment, was identified by Selman Waksman in 1943 and is still used today. Doxycycline and ciprofloxacin are the current first-line choices. If treatment begins within 24 hours of symptom onset for pneumonic plague, or within a few days for bubonic, survival rates exceed 90%. The same disease that swept through medieval Europe killing 6 in 10 is now, in the context of access to antibiotics and rapid diagnosis, essentially curable.
No vaccine is currently licensed for general use in most countries, though candidates exist. The US discontinued its commercial vaccine in 1999 due to limited demand. The plague's continued presence in the world is a story about ecology, not medical inability — we simply cannot reach all the rodent reservoirs.