And bubonic plague, spread by rat fleas, is normally preceded by the death of hordes of rats (only then do the fleas opt for humans). Yet no one in 1348 recorded this.
黑死病由鼠蚤传播，在此之前通常都有大批的老鼠死亡(只有在这时跳蚤才选择人类作宿主)，但在1348年没有人记录下这些。
It is transmitted exclusively when people who have little or no access to safe-drinking water supply swallow water contaminated with parasite-infected fleas.
该病的传播纯属当人们极少获得或者不能获得安全饮用水供应时饮用了含有受到寄生虫感染的蚤类的污染水。
As you watch them jump and hit the top, you will notice something interesting. The fleas continue to jump, but they are no longer jumping high enough to hit the top.
当你观察它们跳跃并且撞到盖子的时候，你会发现一件有趣的事情，跳蚤会继续跳，但是它们不再跳到能撞到盖子的那个高度。
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