20 Times a Pest Changed the Course of Human History

The bubonic plague decimated about one-third of Europe’s population between 1347 and 1351, spreading mainly through the bites of fleas carrying Yersinia pestis. Its consequences disrupted feudal labor structures and shook European religious institutions, and it took generations for the situation to stabilize.

In 1802, Napoleon sent a large army to Haiti to secure a French presence in the Americas. Yellow fever, transmitted by the Aedes aegypti mosquito, killed approximately 80 to 85 percent of his troops. With his ambitions in the New World shattered, Napoleon sold the Louisiana Territory to the United States in 1803 for $15 million.

China kept the secret of silk production hidden for centuries, and the trade routes that developed around this monopoly connected East Asia to Europe, Africa, and India. Goods, religions, languages, and diseases all traveled along these routes. The silkworm laid the foundation for the infrastructure of ancient globalization.

A tiny aphid-like insect, native to North America, arrived in France in the 1860s and began destroying European vineyards, starting with the roots. By 1878, more than 360,000 hectares had been destroyed, and French wine production eventually fell by half. The only solution was to graft French grape varieties onto resistant American rootstocks, a move that forever changed the way wine is produced around the world.

The cotton boll weevil first appeared in Texas from Mexico around 1892 and ravaged the southern United States for three decades, reducing cotton production by more than 50% in some counties. This economic collapse drove hundreds of thousands of Black sharecroppers to migrate north, thereby contributing to the Great Migration, which profoundly transformed urban America.

When Napoleon’s army entered Russia in 1812, body lice spread epidemic typhus through the ranks more quickly than any Russian army could have. Historians estimate that the disease killed far more soldiers than the fighting did, and that the troops that withdrew from Moscow represented only a fraction of those that had entered it.

The tsetse fly, which transmits sleeping sickness to humans and a related disease to livestock, has for centuries hindered certain forms of colonization across sub-Saharan Africa. Vast areas were inaccessible to mounted armies and pastoral agriculture, which shaped the development of societies depending on the regions and parts of the continent that external powers could actually reach.

Locust swarms have been among the most destructive forces in the agricultural history of Africa and the Middle East. In ancient Egypt, massive swarms ravaged the crops of entire regions and destabilized the economy as well as the political order based on agricultural surpluses. A large swarm can consume the equivalent of its own weight in food every day.

Phytophthora infestans, the fungus responsible for late blight in potatoes, destroyed Ireland’s potato crop beginning in 1845 and continuing for several consecutive years. Approximately one million people lost their lives, and at least another million emigrated, mainly to North America, permanently altering the demographics and political culture of both Ireland and the United States.

The French attempt to build a Panama Canal in the 1880s failed largely because malaria and yellow fever, transmitted by mosquitoes, killed tens of thousands of workers. When the United States took over in the early 1900s, the eradication of mosquito breeding grounds was the prerequisite that made the project possible.

The Colorado potato beetle spread across Europe after World War I and eventually devastated potato crops in East Germany, which was then under Soviet occupation. Communist authorities accused the United States of dropping these insects from airplanes as part of a biological attack. Internal reports later confirmed that the infestation was entirely natural in origin, but the propaganda campaign continued for years.

During World War I, body lice spread epidemic typhus among the armies and civilian populations of Eastern Europe, and Serbia lost a significant portion of its population to this disease during the winter of 1914–1915. This epidemic contributed to the destabilization that shaped the postwar political upheavals on the continent.

It is estimated that the Plague of Justinian, which struck the Byzantine Empire beginning in 541 A.D., caused the deaths of 25 to 50 million people over the next two centuries. It considerably weakened the empire just as Justinian was attempting to reunify the Roman world, and many historians consider it a turning point in the history of the early Middle Ages.

Around 550 A.D., according to the historian Procopius, two monks appeared at the court of Emperor Justinian with silkworm eggs hidden inside hollow bamboo stalks, which they had smuggled out of China. The resulting Byzantine silk industry broke the production monopoly that China had held for centuries and ultimately led to the emergence of silk production in Persia, Italy, and throughout Europe.

The locust swarms that struck the Great Plains in the 1930s hit a region already devastated by drought and the Dust Bowl, destroying crops across millions of hectares. Combined with the general collapse of agriculture at the time, these infestations accelerated the abandonment of farming communities and contributed to one of the largest internal migrations in U.S. history.

In Enterprise, Alabama, the destruction of cotton crops by the cotton boll weevil forced local farmers to diversify by turning to peanuts and other crops, which proved to be more profitable than cotton. In 1919, the city erected a monument in honor of this pest, which still stands today. It is believed to be the only monument in the world dedicated to an agricultural pest.

Yellow fever profoundly transformed New Orleans on several occasions during the 18th and 19th centuries, decimating the population during massive epidemics and determining which communities could survive the city’s summers. These epidemic cycles influenced the choice of immigrant groups that settled there and directly shaped the city’s unique social and political structure.

Food security in the Roman Empire depended on the Annona, the public system responsible for supplying the city with grain, and rats posed a constant threat to it. The damage caused by rodents in warehouses and during transport was so extensive that the Romans officially kept cats in their granaries to control pests, marking one of the earliest documented institutional uses of the domestic cat.

The third global plague pandemic, which began in China in the 1850s and spread via maritime routes, claimed millions of lives across Asia and created permanent foci of the disease, from India to the southwestern United States. It also directly accelerated the scientific research that led to the identification of Yersinia pestis as the causative agent in 1894, thereby laying the foundation for the modern understanding of the spread of infectious diseases.

The tumor fly laid its eggs in the open wounds of livestock, and the larvae gnawed at the living tissue, often leading to the animal’s death. In the mid-20th century, this scourge cost the U.S. livestock industry hundreds of millions of dollars a year. In the 1950s and 1960s, the U.S. Department of Agriculture (USDA) succeeded in eradicating it from the United States by releasing hundreds of millions of sterile male flies.