I will be showing you some vivid material,

我要给你们展示几个生动的材料
and I'm sorry that it's just before lunch.

很抱歉这堂课正好在午饭前
And indeed a couple of years ago in this class

几年以前在这门课上确实有一位
someone in the front row fainted with some of the pictures.

坐在前排的同学在看到这些图片时晕倒了
So, if you don't wish to look at them,

如果你们不希望看到这些图片
I can tell you in advance it's not compulsory.

事先声明这些图片不是你们必须要看的
You can simply close your eyes.

你可以闭上眼睛
In any case, yersinia pestis is exceptionally virulent

总之鼠疫耶氏菌是绝对致命的
because of its ability to overwhelm the immune system of the body.

因为它有摧毁人体免疫系统的能力
After an infective bite, there's an incubation period,

被传染源咬伤后会有一段潜伏期
usually of one to six days,

潜伏期一般是一至六周
and then the classic symptoms appear-- the first beginning--

然后会出现一些典型症状最初