首先指出popilarize应该是你拼错了吧,应该是popularize.
He helped popularize the idea that后面跟的是一个同位语从句,一直到句子结尾.
Some diseases not previously thought to have a bacterial cause
were actually infections可以看作一个完整句子,里面含有一个定于从句:Some diseases (that were) -not previously thought to have a bacterial cause-定于从句修饰成分,
were actually infections.
同位语从句的内容翻译过来就是:一些本来不被认为由细菌引起的疾病事实上是由感染引起的.(感染就是因为有细菌).
所以答案是正确的.希望你能从我的回答中有所收获