natural focus的用法和樣例：
例句
- Conclutions The virulence of Y.pestis from plague nature foci is moderate.
結論河北省鼠疫自然疫源地病原菌毒力屬于中等毒力。 - As one of the oldest identifiable diseases known to man, plague remains endemic in many natural loci around the world.
鼠疫作為人所共知的一種最古老可確定的疾病，在世界上許多自然疫源地仍呈地方性流行。
