许多儿童疫苗依赖于“群体免疫”——一定比例的人口接种某些疫苗是为了预防那些没有接种或不能接种的人受到感染。
Many of the childhood vaccinations rely on "herd-immunity" - a certain percentage of the population having the vaccine in order to protect those that do not, or cannot have it, from infection.
一些猴子死亡，至少有四人受到感染，虽然他们都没有出现临床疾病。
Several monkeys died, and at least four people were infected, although none of them suffered clinical illness.
美国农业官员指出，美国只发生过两起记录在案的疯牛病病例，而且受到感染的牛肉并没有进入食品供应系统。
American agriculture officials point out there have only been two documented cases of mad cow disease in U.S. cattle - neither of which made it into the food supply.
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