求翻译：jhon snow was a famous doctor in london-so expert,indeed ,that he attended queen victoria as her personai physician.but he became inspired when he thought about helping ordinary people exposed to cholera.this was the deadly disease of its day.neither its cause nor its cure was understood.so many thousands of terrified people died every time there was an outbreak john snow wanted to face the challenge and solve this problem.he knew that cholera would never be controlled until its cause was found.是什么意思？

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