This passage is effective in supporting Roosevelt's argument because it compares aggressive natures to something that everyone can agree must be contained: a dangerous physical disease.
The passage belongs to the Quarantine Speech, given by President Franklin Delano Roosevelt on October 5, 1937. In the speech, Roosevelt called to place Germany, Italy and Japan under political "quarantine", comparing the spread of international violence with the spread of a disease.
With this speech, Roosevelt called for an end to American isolationism and the appeasement policy of Britain and France.