18. For a social epidemic to occur, however, each person so affected must then influence his or her own acquaintances, who must in turn influence theirs, and so on; and just how many others pay attention to each of those people has little to do with the initial influential.

【分析】并列复合句。分号连接两个分句。第一个分句的主干为 each person so affected must then influence his or her own acquaintances，后面 who 引导的定语从句修饰 acquaintances。 句中 For a social epidemic to occur 为状语，so affected 是 each person 的后置定语。第二个分句中，疑问词 how many 引导的句子作主语，谓语为 has little to do with，宾语为 initial influential。

【译文】不过，要出现某个社会潮流，每个被这样影响的人再去影响他们认识的人，如此循环下去。其他人中有多少注意到这些人，几乎与最初的影响者没有什么关系。

19. If people in the network just two degrees removed from the initial influential prove resistant, for example, the cascade of change won't propagate very far or affect many people.

【分析】句子主干为 the cascade of change won't propagate very far or affect many people。if 引导条件状语从句，主语是 people，谓语是 prove， resistant 为表语，in the...influential 是 people 的后置定语。for example 为插入语。

【译文】例如，如果在这个（相互影响的）网络中，距离最初影响者仅有两个级别远的人就具有抵抗性，那么这一连串的变化就不会传播太远或影响太多人。

【点拨】1) removed 本句中用作形容词“远离的”，（be） removed from “远离...的”。2) cascade 名词意为“小瀑布；瀑布状物；倾泻”，动词意为“倾泻；大量垂悬”，本句中是要表达“一连串”的意思。3)propagate“繁衍；传播；宣传”，句中意为“传播”。