The widespread, uncontrolled spread or proliferation of something (e.g., disease, pest, bad practice); often used as '蔓延する' (to spread rampantly).
インフルエンザが市内で蔓延しているので、外出を控えてください。
Influenza is spreading throughout the city, so please refrain from going out.
不正行為が業界に蔓延すると、信頼を取り戻すのは難しい。
If dishonest practices become widespread in an industry, it is hard to restore trust.
雑草が庭に蔓延して、手入れが大変だ。
Weeds have spread throughout the garden, making maintenance difficult.