The first is that, when we started trying to eradicate polio about 20 years ago, more than twice as many countries were infected than had been when we started off with smallpox.
首先 我们开始尝试根除脊髓灰质炎时 时间大概是20年前 当时，被其感染的国家数量 是开始根除天花之际的两倍多