Pestilence is a deadly disease that spreads rapidly through a population, killing many people. Throughout history, pestilences have killed millions: the Black Death swept through medieval Europe in the 1300s, killing perhaps a third of the population. Smallpox, typhus, and cholera have all been called pestilences when they struck communities with devastating force.

The word carries a sense of overwhelming catastrophe. While we might call a bad flu outbreak an epidemic, we reserve pestilence for diseases that bring widespread death and social breakdown. When pestilence struck a medieval city, normal life stopped: markets closed, families fled, and survivors struggled to bury the dead.

You might encounter this word in historical accounts or in older literature. In the Bible, pestilence appears as one of the great calamities that can befall humanity. The word sounds archaic because modern medicine has made true pestilences rare. Vaccines, antibiotics, and public health systems now prevent or control diseases that once devastated entire regions.

The related word pestilent means harmful or destructive, though it's sometimes used more lightly, as when someone calls a swarm of mosquitoes pestilent insects.