Why did Roosevelt compare war to a disease in his quarantine speech

The quarantine speech was presented by president Roosevelt where he called the positive endeavours to quarantine the land hungry dictators. He did not directly mention any country, although the speech was focused on the Nazi Germany, the Japan's empire, and the Kingdom of Italy.

He mentioned in his speech that the spread of violence and war is like a contagious disease, he said that, when a few members get infected by a contagious disease, the country tries to destroy them, the same treatment should be done with them who try to break a country's peace through violence.