The SIS model departs fundamentally from the previous cases. Recovery does not confer immunity. Instead, recovered individuals return to the susceptible compartment. For that reason, the system does not produce a closed epidemic wave. What emerges instead is a gradual convergence toward an endemic equilibrium. The red infection curve rises and stabilises at a persistent level, while the green susceptible curve declines without vanishing. That equilibrium is the defining feature of the SIS model.
