History is often said to hinge upon great battles, both in war and among competing ideas. But some battles are fought on the microscopic scale. Bacteria, parasites, and viruses have swept through cities and devastated populations, felled great leaders and thinkers, and in their wake transformed politics, public health, and economies. For example:
Influenza. The disease influenced the course of World War I by sickening and killing soldiers and straining military healthcare systems. Some have said that President Wilson's negotiations during the Treaty of Versailles were affected by the influenza infection he had at the time.
Cholera. Spread via paltry or nonexistent sewage systems and lack of clean water, it led to improvements in sanitation in many places throughout the world.
Yellow fever. This disease influenced the building of the Panama Canal, the Louisiana Purchase, and the pre-World War II development of the southern United States; in the South, populations were decimated or debilitated by the disease.
Hemophilia and porphyria. These genetic blood disorders had serious effects on the crowned heads of Europe. The rise of Spanish dictator Francisco Franco can be traced to the lack of an heir to the throne because of hemophilia, and the collapse of the Romanov dynasty in Russia was in part due to hemophilia. Some believe that George III's treatment of his American subjects, which helped to trigger the American Revolution, was in part affected by his porphyric attacks.
Potato blight. Potato blight devastated a staple food that fed much of Ireland in the mid-1800s. The Irish famine influenced America by generating an influx of Irish immigrants to U.S. cities.
For information about the influence of other diseases on history, click the link below.