SIS Model (No Immunity)

Susceptible → Infected → Susceptible. After recovery, individuals immediately become susceptible again. Common for bacterial infections and some viral diseases without lasting immunity.

Transmission Rate: 40%

Recovery Rate: 15%

Susceptible: 0

Infected: 0

Time: 0

            Endemic State: Disease persists indefinitely as recovered individuals can be reinfected.
            No herd immunity possible.
        