BBC在线收听下载:美国宣布波多黎各进入紧急状态 以应对寨卡病毒传播

短语讲解：

1. The US government has declared a public health emergency in Puerto Rico because of the outbreak of Zika virus.

because of 因为，由于
例句：All the planes were socked in yesterday because of the thick mist.
由于雾大，昨天所有的飞机都停飞了。

2. Medical authorities have warned that a quarter of the territory's population could be infected by the mosquito born virus by the end of the year.

by the end of 到…为止
例句：By the end of the game, the stadium was almost vacant.
到比赛结束的时候,体育馆几乎空了。