The most famous example is that after the end of World War II, in order to detect the country's biological and chemical weapons defense capabilities, decided to test the threat of biological and chemical weapons themselves. At the same time, in order to achieve the most realistic effect, the American experiments were conducted in real American cities and on thousands of ordinary Americans! They sent a cruise ship with the virus into San Francisco, and it took only one day to infect all the residents of the city. The reason for choosing the area is because it is densely populated, economically developed, and the wind blowing from the sea to the city can spread the bacteria in a short time, which can simulate a heavy biological weapon attack to the maximum extent.