1 . 根据课文内容，在空白处填入所缺的单词。

       However, he never lost his desire to destroy cholera once and for all. In     1    , doctors in those days had two     2     theories to explain how cholera spread. One theory was that bad air caused the disease. Another was that cholera was caused by an     3     from germs in food or water. Snow     4     to the second theory. It was correct, but he still needed     5    .