So, you are probably waiting for a big red spike like before. But guess what? It never comes. In the SIS model, people who recover become susceptible again. That means the disease does not simply disappear. Instead, it settles into a steady pattern. The red curve rises slowly and then stays at a stable level. At the same time, the green curve drops but never reaches zero. In the end, both lines run almost in parallel. That is the balance point where the disease remains in the population.
