A plague is a widespread disease that spreads quickly through a population, causing severe illness and often death. The most famous was the Black Death, a plague that killed roughly one-third of Europe's population in the 1300s. Before modern medicine, plagues were devastating because doctors didn't understand how diseases spread or how to treat them.

When we say something spreads “like the plague,” we mean it spreads rapidly and uncontrollably.

Today, we also use plague as a verb meaning to cause continual trouble or distress. A student might be plagued by math problems they can't solve, or a town might be plagued by potholes. In this sense, the word captures that feeling of being overwhelmed by persistent problems.

Scientists can now control many plagues through vaccines, antibiotics, and public health measures. Large outbreaks are now rare, though small cases still occur. Understanding how plagues spread helped scientists develop the germ theory of disease, which revolutionized medicine and saved countless lives. When someone says “avoid it like the plague,” they mean stay far away from something dangerous or unpleasant.