The SIR (susceptible-infected-removed) model, developed by Ronald Ross1,
William Hamer, and others in the early twentieth century [4], consists of a
system of three coupled non-linear ordinary differential equations, which does
not possess an explicit formula solution. However, simple tools from calculus
allow us to extract a great deal of information about the solutions. Along the
way we illustrate how this simple model helps to lay a theoretical foundation
for public health interventions and how several cornerstones of public health
required a similar model to discover.