泛滥成灾 become rampant and cause disaster
Explanation
泛滥成灾，指洪水泛滥成灾，比喻不好的事物像洪水一样泛滥，造成灾害。
It literally means "to overflow and cause disaster." Figuratively, it describes the spread of harmful things like a flood, resulting in disasters.
Origin Story
很久以前，黄河经常泛滥，每当雨季来临，河水便会冲破堤坝，肆虐四方。庄稼被淹没，房屋被冲毁，人们流离失所，哀鸿遍野。一场大雨过后，河水暴涨，瞬间冲垮了沿岸的村庄，人们惊恐万分，四处逃窜。这场洪水持续了数日，给人们带来了巨大的灾难，无数的人们家破人亡，流离失所，这便是泛滥成灾的真实写照。
Long ago, the Yellow River often flooded. Every time the rainy season came, the river water would break through the dykes and wreak havoc everywhere. Crops were submerged, houses were destroyed, and people were displaced and suffered. After a heavy rain, the river water surged, instantly destroying the villages along the banks. People were terrified and fled everywhere. This flood lasted for several days, bringing great disaster to people, countless people lost their homes and families, and became displaced, this is a true portrayal of rampant disaster.
Usage
泛滥成灾多用于形容不好的事物泛滥成灾，如谣言、网络暴力等。
The idiom is mainly used to describe the spread of negative things, such as rumors, cyberbullying, etc.
Examples
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网络暴力泛滥成灾，严重影响社会秩序。
wǎngluò bàolì fàn làn chéng zāi, yánzhòng yǐngxiǎng shèhuì zhìxù
Cyberbullying is rampant, seriously affecting social order.
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谣言泛滥成灾，误导了很多人。
yáoyán fàn làn chéng zāi, wùdǎole hěn duō rén
Rumors are rampant, misleading many people.