Smallpox, on the other hand, it's over three.

天花超过三个
Now, before we had a vaccine...

在我们研发出疫苗之前
...polio spread at a rate between four and six.

小儿麻痹患者通常能感染四到六个人
Now, we call that number...

现在我们将这个数字称作
...the R-nought.

基本传染数
R stands for the reproductive rate of the virus.

传染数是指病毒的繁殖率
Any ideas what that might be for this?

跟这事有什么关系吗
How fast it multiplies depends on a variety of factors.

病毒增殖的速度取决于一系列因素
The incubation period, how long a person is contagious.

潜伏期一个人被感染了多久
Sometimes people can be contagious without even having symptoms.

有时候被感染的人不会有任何症状
We need to know that too.

这一点我们也需要知道
And we need to know how big the

我们还需要知道