英語時差：疫苗的神奇作用B

The second type of vaccine contains an attenuated virus. This virus is alive, but has been grown in the lab for several generations, so it isn’t able to multiply very well in humans. With the attenuated virus multiplying very slowly, your body is able to produce the correct antibodies before you get sick. Measles and mumps vaccines are both made with attenuated viruses. When most people in a community have become resistant to the virus, either through a vaccination or by having the virus and recovering, the community develops what is called “herd immunity”. The nice thing about herd immunity is that it becomes statistically less likely for the germ to find a person without immunity. This protects members of the community who haven’t been vaccinated, or had the disease, by reducing their chances of encountering an infected person.