In this model, this model tracks the evolution of three sub-populations:

* S(t) = susceptible individuals at time t
* I(t) = infected individuals
* R(t) = recovered individuals

For a general Petri net, these are called _species_.

There is a process called _infection_, which takes as "inputs" one infected individual and one susceptible individual, and "outputs" two infected individuals. This process can be written:

\\[\mathit{infection}: S + I \rightarrow I + I\\]

Another process called _recovery_ inputs an infected individual and outputs a recovered individual:

\\[\mathit{recovery}: I \rightarrow R\\]

The processes are also referred to as transitions, or reactions.

Each process has an associated _transition rate_, which is a kind of "speed coefficient" for the reaction.