The Black Death began in Central Asia, most likely in the mountains of what is now Kyrgyzstan, around 1338. From there, the bacterium Yersinia pestis spread along trade routes westward, reaching the Black Sea by 1346 and the ports of Southern Europe by 1347. Over the next four years, it killed more than 50 million people in Europe alone.
The Outbreak Traced to Kyrgyzstan
For centuries, historians debated exactly where the pandemic originated. That question was largely settled in 2022, when researchers extracted ancient DNA from the teeth of people buried in two cemeteries in northern Kyrgyzstan. These individuals died in 1338 and 1339, roughly a decade before the plague devastated Europe. The researchers reconstructed the full genome of the plague bacterium from those remains and compared it against 203 modern plague samples and 47 historical samples dating from the 14th to 18th centuries.
The results were striking: the strain that killed those people in Kyrgyzstan was a direct ancestor of the four plague strains previously linked to the Black Death in Europe. In other words, the pandemic’s family tree traces back to a single point in the Tian Shan mountains of Central Asia.
Wild Rodents Carried the Bacterium for Centuries
Yersinia pestis didn’t appear out of nowhere in the 1330s. The bacterium had been circulating quietly in wild rodent populations across Central and East Asia for a long time. In Mongolia, the Siberian marmot has historically been the most common source of human plague infections. Several species of gerbils, including the great gerbil and the Mongolian gerbil, also serve as natural reservoirs, carrying the bacterium without always dying from it. These animals live in burrows across the vast grasslands and semi-arid regions of Central Asia, and the fleas that feed on them keep the bacterium cycling through the population.
Under normal conditions, this cycle stays contained in remote areas with little human contact. Something had to change to push the bacterium out of its usual territory and into human trade networks.
Climate Shifts Set the Stage
That something was likely the climate. The early 14th century marked a transition from a warm period into the Little Ice Age, bringing years of unusually heavy rainfall starting around 1315. According to research from NASA’s Jet Propulsion Laboratory, this sustained increase in precipitation boosted plant growth across the Eurasian steppes and deserts, which in turn fueled a rodent population explosion. More food meant more rodents, and more rodents meant more fleas.
The cool, humid conditions also favored flea survival and breeding. At the same time, the disruption of pastoral herding in Central Asia (due to political upheaval and climate stress) reduced competition for food from domesticated livestock and meant fewer hunters keeping wild rodent numbers in check. Wild rodents carrying plague spread outward from their usual habitats and came into contact with domestic rats living closer to human settlements and trade routes. That crossover from wild rodent to domestic rat was the critical step that put the bacterium on a path toward Europe.
How Fleas Spread the Infection
The plague spreads primarily through flea bites, and the biology behind it is grimly efficient. When a flea feeds on an infected rodent, it ingests the bacterium along with the blood. Inside the flea’s gut, Yersinia pestis multiplies and forms a dense, sticky mass called a biofilm in the proventriculus, a tiny valve in the flea’s foregut that normally controls the flow of blood into the digestive system.
As the biofilm grows, it partially or completely blocks this valve. Blood can no longer pass through to the flea’s stomach. When the flea bites its next host (a rat or a human), it tries to feed but the incoming blood hits the bacterial blockage and gets pushed back out into the bite wound, carrying plague bacteria with it. The blocked flea is essentially starving, so it bites repeatedly and aggressively, increasing the chances of transmission with each attempt. This mechanism is what made plague so explosively contagious once it entered rat populations living alongside humans.
The Path From Central Asia to Europe
From Kyrgyzstan, the plague moved westward along the Silk Road trade networks that connected Central Asia to the Black Sea. Infected rats and their fleas traveled with caravans of goods, spreading the disease from town to town. By 1346, the plague had reached the port city of Caffa (modern-day Feodosia) on the Crimean Peninsula, where Mongol forces were besieging a Genoese trading colony.
A 14th-century account by the Genoese notary Gabriele de’ Mussi describes the Mongol army catapulting plague-infected corpses over the city walls, which would make it one of the earliest recorded attempts at biological warfare. Historians consider this account plausible, though they also note that plague likely entered Caffa through other means as well, such as rats moving freely around and through the siege lines. The entry of plague into Europe from the Crimea probably would have happened regardless of the catapulted corpses.
Genoese merchants fleeing Caffa carried the infection aboard their ships. By late 1347, plague-bearing vessels had arrived in Messina, Sicily, and soon after in Genoa and other Mediterranean ports. From these coastal cities, the disease radiated inland with terrifying speed, following river routes and trade roads deeper into the continent.
Three Forms of the Disease
The plague struck the body in three distinct ways, each progressively more dangerous. The most common form was bubonic plague, named for the painful, swollen lymph nodes (called buboes) that appeared in the groin, armpit, or neck within days of a flea bite. Even without treatment, roughly 40% to 70% of people with bubonic plague survived, though in the crowded, malnourished conditions of 14th-century Europe, survival rates were likely worse.
If the infection spread to the bloodstream, it became septicemic plague, causing organ failure and dark discoloration of the skin from dying tissue. This form carried a fatality rate between 30% and 100% without treatment. The deadliest form was pneumonic plague, which developed when the bacteria reached the lungs. Pneumonic plague was always fatal if untreated and, crucially, could spread directly from person to person through coughed droplets. This meant that in dense urban areas, the disease no longer needed fleas or rats to keep spreading.
Between 1346 and 1351, the Black Death killed more than 50 million people in Europe, wiping out roughly a third to half of the continent’s population. The combination of an aggressive bacterium, an efficient flea-based transmission system, climate conditions that supercharged rodent populations, and a connected network of trade routes created a catastrophe that reshaped European society for generations.