With the default values, run the simulation a few times to see what happens - you need to point at a green dot and click on it to start an infection [you may have to make a few attempts to catch one properly!].
Here you have rather more things you can control [the parameters of the simulation]. Try varying them in a systematic way e.g. change the size of the population and see if that makes a difference, and if so what it is.
Now change another e.g. the infectious period and see if that makes a difference.
Now try changing the size of the population for different infectious periods and see if this affects the behaviour in a consistent way, and to the same extent each time.