We first examine the effect of disease mortality in the *I*_{1} class (parameter *ρ*). As relative disease mortality in *I*_{1} increases, the ESS virulence decreases. The degree of decrease in ESS virulence depends on the parameter scaling virulence relative to transmission (*ψ*, *a*). As the amount of transmission increases per unit of virulence, the ESS virulence decreases. The decrease in ESS virulence is greatest when there is no disease mortality in *I*_{1} (*ρ* = 0; *a*).

The relative amount of transmission in *I*_{2} (*ϕ*) also has a large effect on ESS virulence (*b*). As the amount of transmission in *I*_{2} increases, the ESS virulence decreases, and the rate of decrease depends on the level of mortality that occurs in the *I*_{1} class. As the level of transmission in *I*_{2} and the disease mortality rate in the *I*_{1} class (*ρ*) approach zero, the ESS virulence goes to infinity (*b*). These results can be understood by realizing that for any fixed level of virulence (*α*), decreases in the transmission parameter *ϕ* reduce the fitness benefit of reaching the second class (*I*_{2}), while increases in *ρ* both decrease the probability of reaching the second class and decreases the infectious period in the first class. Therefore, as both parameters reach zero, there is no benefit in reaching the second class and no cost to virulence in the first class. Thus, ESS virulence is very high and virulence will have a greater tendency to increase after introduction.

The effect of the transition rate (*γ*) between the classes on ESS virulence depends strongly on the relative amount of disease mortality in *I*_{1} (*ρ*) and on the recovery rate (*σ*; ). When disease mortality in the classes is similar (*ρ* = 1) and there is recovery from the second class (*σ* = 10), ESS virulence increases with increases in transition rate between the classes (*a*). When there is no recovery, however, there is no change in ESS virulence with transition rate (*b*). This result is because of the pathogen, on average, spending relatively more time in the *I*_{1} class as *γ* decreases. In the limit as *γ* goes to zero, the pathogen spends its whole life in *I*_{1} and has an infectious period of 1/(*μ* + *α*). Thus, pathogen virulence adapts to *I*_{1}, and reductions in virulence translate directly to a longer infectious period. As *γ* increases, however, the pathogen spends little time in *I*_{1} and, in the limit, experiences an infectious period of 1/(*μ* + *α*+ *σ*). Thus, the pathogen adapts to the conditions of the *I*_{2} class and to a higher virulence as long as *σ* > 0.

With less disease mortality in *I*_{1} (*ρ* < 1), ESS virulence first increases and then decreases as transition rate (*γ*) increases from zero (*a*,*b*). The degree of increase, and the inflection point where ESS virulence begins to decrease, depends on the amount of disease mortality in *I*_{1} (*ρ*, ). At the extremes of *γ* (0, ∞), the ESS virulence converges to that of a model with one infectious stage, just as described above, but here the ESS virulence is always higher for low values of *ρ* and *γ* ().