We know now that it's caused by a related virus, called vaccinia, remember the virus that causes smallpox is called variola.

我们现在知道这病由一种相关的病毒引起,这种病毒叫牛痘病毒,想想天花病毒也被叫做牛痘病毒

耶鲁公开课 - 生物医学工程探索课程节选

It's reproduction of the virus and passage of that virus onto new cells which causes the problems with disease that we associate.

病毒一旦繁殖并进入新细胞,就会带来相关病症的危害

耶鲁公开课 - 生物医学工程探索课程节选