蔓延 (mànyán) is a verb meaning 'to spread'—especially in an uncontrolled, creeping, or pervasive way, like vines growing over a surface. The first character 蔓 (màn) means 'vine' or 'creeping plant', evoking organic, horizontal expansion; the second character 延 (yán) means 'to extend' or 'to stretch out'. Together, they form a vivid compound suggesting gradual, often unstoppable diffusion across space or through a system.

This word is commonly used for negative or neutral phenomena: diseases, rumors, fire, pollution, or emotions spreading through a group. It emphasizes continuity and coverage—not sudden bursts, but steady, expanding influence. While it can describe literal botanical growth (e.g., ivy spreading up a wall), its most frequent use is metaphorical, especially in news reports, public health announcements, and literary descriptions of social or environmental issues.