So they kept track of country-by-country, when cases of smallpox occurred, and as it was eliminated from regions of the country, they then certified those regions to be smallpox free.

它跟踪调查了一个又一个国家,记录天花在这个国家中何时发生,又是何时清除的,并确证这个区域已经没有天花

耶鲁公开课 - 生物医学工程探索课程节选