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

So, some of these cases were probably due to polio that was transmitted by incompletely killed virus that was present in the vaccine.

因此,那些病例的发病的原因,是被那些疫苗中携带的,没有被完全杀死的病毒感染

耶鲁公开课 - 生物医学工程探索课程节选

Right, but if you just got the primary exposure, wouldn't the secondary response automatically-- not a booster shot, just the secondary response, because you were exposed?

但这时你已经获得这样的初次免疫,而因为你已经天然感染过该病毒,那么,在没有加强针的情况下,二次免疫是否无法自动发生

耶鲁公开课 - 生物医学工程探索课程节选

Now, a virus is not capable of reproducing on its own, so if you got a virus into your body somehow and it didn't enter any of your cells, it would cause no damage because it could not reproduce.

病毒自身并不能繁殖,所以如果你的身体感染了病毒,而病毒没有进入细胞的话,因其不能繁殖所以不会对身体造成伤害

耶鲁公开课 - 生物医学工程探索课程节选