Because if it was a disease that was carried by both humans and squirrels, for example, then in order to get rid of it on the planet, you'd have to vaccinate not only all the people but all the squirrels, right?

因为假如人和松鼠都能携带,某种疾病的话,想要把这种疾病从地球上清除,不仅要给所有人接种疫苗,还要给所有松鼠接种疫苗

耶鲁公开课 - 生物医学工程探索课程节选