For this calculation we assume that a person is infective for roughly seven days, and

that the incidence - the number of registered infections within the last seven days person

100.000 - is a near enough approximation to this. If you have other assumptions, just

adjust the incidence to the number of infected per 100.000 you assume.

Another assumption is that each member of the population has the same base chance

to be infected, based upon the incidence. This is rarely so, but hard to simulate otherwise.

If you are not happy with this, adjust the incidence or make your own script :-)