鼠疫 (shǔ yì) literally means 'rat plague' — 鼠 (shǔ) means 'rat' or 'mouse', and 疫 (yì) means 'epidemic' or 'plague'. Historically, it refers specifically to bubonic plague, a deadly infectious disease caused by the bacterium Yersinia pestis, which is transmitted primarily through fleas that infest rodents. Because rats were central to its spread in past outbreaks — especially during the Black Death in medieval Europe and major epidemics in 19th- and early 20th-century China — the name directly reflects this ecological link.

Today, 鼠疫 is a formal, medical, and historical term used in public health contexts, academic writing, and news reports about disease surveillance. While rare in modern times due to antibiotics and sanitation, it remains a notifiable disease in China and globally. It is never used colloquially for minor illnesses — doing so would cause alarm or confusion, as it carries strong connotations of high fatality and epidemic potential.