语法填空（阅读下面材料，在空白处填入所需的内容或括号内单词的正确形式，不多于3个单词
John Snow was a well-known physician in London. At that time, cholera was        deadly disease and thousands of people died of it. People didn’t know its cause or its    . John Snow determined to face the     and solve the problem.       another outbreak hit London in 1854, he gathered information and made a map,       (mark) the exact places where all the dead people had lived. Through further investigations, he discovered that the water was to    , because it was polluted by the dirty water from London.       supporting evidence, John Snow announced that polluted water carried the germs. To prevent this from        again, John suggested that the source of all the water supplies        (examine) and new methods of       (处理) polluted water be found. Thus cholera was defeated in the end.