Remember that the lucky thing about the smallpox vaccine was that a naturally occurring attenuated form of the smallpox virus variola called vaccinea was found.

对于天花病毒来说,幸运的是,存在一种自然发生的,被称作痘苗病毒的,天花病毒的衰减疫苗

耶鲁公开课 - 生物医学工程探索课程节选