難句解析：
①"But", he cautions, "some people may have the idea that once one understands the causes, the cure will rapidly follow. Consider Pasteur, he discovered the causes of many kinds of infections, but it was fifty or sixty years before cures were available."
▲此句是一個直接引語，注意引語中有兩個句子，前面的句子有一個復雜的同位語從句that once one understands the causes, the cure will rapidly follow修飾the idea，而后面提到了一個例子，句中有一個but，它后面的it指代前面整句話。
△注意引語中兩個句子之間的關系，后者是用來駁斥前面一句中那些人的錯誤想法的，作者在這里舉的是一個反例。
②The researchers made great progress in the early 1970s, when they discovered that oncogenes, which are cancer-causing genes, are inactive in normal cells.
▲此句主句是The researchers made great progress in the early 1970s，后面有一個when引導的定語從句修飾the early 1970s，而在這個定語從句中還有一個賓語從句，在賓語從句中又有一個非限定性定語從句which are cancer-causing genes修飾oncogenes。
△注意此句中有超綱詞oncogenes，但是我們可以從后面修飾它的定語從句推出其確切含義，這也是常用到的閱讀方法之一，即可以用定語從句來反推所修飾詞的意思。
③The exact mechanisms involved are still mysterious, but the likelihood that many cancers are initiated at the level of genes suggests that we will never prevent all cancers.
▲此句是用but連接的復合句，注意前面的involved表示“相關的”，而后面的半句的主干是the likelihood suggests that...，里面又包含有一個同位語從句和賓語從句。
△注意看清句子結構，而后面的likelihood表示“可能性”。
④Environmental factors can never be totally eliminated; as Hayward points out, "We can't prepare a medicine against cosmic rays."
▲此句用分號隔開，前面是一個簡單的被動語態，而后面是一個直接引語。
△如果我們了解分號的用法，就知道它經常用來連接兩個意思相仿或相對的句子。而在這里，分號后面的部分就是進一步說明前面的意思，那么我們也就知道cosmic rays是environmental factors的一個個例。
來源:可可英語 http://www.ccdyzl.cn/kaoyan/201408/317070.shtml