Historical plagues are widespread outbreaks of infectious diseases that have caused significant mortality throughout human history. Notable examples include the Black Death in the 14th century, which killed millions in Europe, and the Spanish Flu of 1918, which infected about one-third of the world's population. These events often led to social, economic, and cultural changes. Plagues are typically caused by bacteria or viruses and can spread rapidly in crowded or unsanitary conditions. The bubonic plague, caused by the bacterium Yersinia pestis, is one of the most infamous, while other diseases like cholera and smallpox have also had devastating effects on populations worldwide.