You infect them with the virus, and you let the cells, in culture, produce the virus.

再用病毒感染它们,使培养基中的细胞产生病毒

耶鲁公开课 - 生物医学工程探索课程节选

And those nine people make three-- each one-- three other people smile.

那九个人-,每个人再用笑容感染三个人。

哈佛公开课 - 幸福课课程节选