Cured individuals have had the disease, and are now immune. The numbers in each group will change from day to day, and the total population will decrease as people die. If the time span is short enough, births of new individuals can be disregarded. The number of people in the susceptible group will decrease in each time period as individuals in that group get sick. You may not like the assumption that every susceptible person encounters every sick person once per day.
P is the rate of contagion per encounter .
The number of people in the sick group will increase by the number of susceptible people that get sick. The population will always be decreasing proportional to the number of sick individuals times the death rate. Building the Model A spreadsheet model for this scenario can be built similar to the others we have seen. Time will proceed increment by increment down the columns. Put the three rates A, B, C in three cells off to the side where they can be conveniently changed.