This program is intended to simulate, in a simplified but rather realistic way, how a contagion may spread within a community. The individuals are subdivided into: healthy (green), asymptomatic (yellow), sick(red), immune (light blue). The contagion is transmitted, with different probabilities, through contact with an asymptomatic or sick individual. Asymptomatic individuals can develop the illness, but not necessarily. Sick individuals can either die, in which case a black dot is displayed for a fraction of second, or get out. Asymptomatic and heald individuals will eventually become immune, but just for a while (either for a decrease of their anticorps, or due to virus mutation). Another detail: sick people move slower than the other ones. The velocity parameter is tightly related to the probability of encounters. It is the only parameter that can be changed in real time, with a slider, while all other parameters, about the infectivity, mortality, mean duration of the disease as well as of the immunity, etc. can be changed in the code (comments have been added in it, to make easier to identify the various parameters). I recommend to try different choices of parameters, in order to get a feeling of their effects. Changing the value of the Velocity is equivalent to add or release social restrictions. One may notice how changes have a delayed effect on the percentage of sick people and, even later, on the rate of deaths. The program ends when only healthy (and immune) individuals are left, and a snapshot of the final situation is displayed to a few seconds. The initial number of individuals is set to 300 (just because of a limitation of Scratch on the maximum amount of clones): due to this limited statistics, trends and percentages are rather unstable, but the final trends can be noticed anyway. Incidentally, all times for changes to status are not fixed, but follow Poissonian distributions, like in the real world. Finally, the use of clones, as well as the condition of a sprite touching a given color, are very useful programming tools to easily code a rather complex evolution.
