What if you're talking about a group of people that don't have any cows in their vicinity, let alone a sick cow?

万一你遇到一群人,他们附近连头奶牛都没有,更不用说受病毒感染的奶牛

耶鲁公开课 - 生物医学工程探索课程节选

And that was possible in smallpox because of a few characteristics of the disease: one is that it's a--it's purely a human disease, humans are the only organisms that are affected by it.

天花能够被根治的原因在于,这种疾病具有以下几个特点,一是,它是一种只有人类才得的疾病,人类是唯一受天花感染的生物

耶鲁公开课 - 生物医学工程探索课程节选

Children are particularly influenced by the contagiousness of laughter, but just in general, if you hear somebody laughing, like the kids you saw before, it's the sort of thing that could easily make you laugh.

孩子特别容易,受笑声的感染，但普遍来说，如果你听到有人笑，像你们刚才看到的小孩子，这样的事物很容易引你大笑。

耶鲁公开课 - 心理学导论课程节选