Table 1.

State variables for the model specified in figure 1 and equation (3.1). All variables represent numbers of individuals.

state variable	meaning
S	susceptible to infection
$I_{\mathrm{m}}$	mildly infectious (variolated)
$I_{\mathrm{s}}$	severely infectious
$R_{\mathrm{m}}$	removed, after mild infection
$R_{\mathrm{s}}$	removed, after severe infection