05.06.2026 | Spreading phenomena

An epidemiologist observed the behavior of a new disease that emerged in southern Brazil 3 years ago. The data shows that 15% of a city's population always has the active virus at any time of the year. The figure below demonstrates the fraction of infected individuals over time. Which endemic model does the new disease follow?

A. It is a disease following the SIR model, where, although part of the population builds resistance, in this model there is a transmission resistance to the outside of the city and it will always be present, even if the entire population is affected.
B. It is a disease following the SIS model, in which individuals recover from the illness but do not develop permanent immunity, returning to the susceptible state. This continuous cycle allows the virus to circulate indefinitely, balancing the infection and recovery rates to maintain a stable endemic state of 15%. 
C. It follows the SI model, given that the virus remains active year-round. In this scenario, the 15% represents the natural maximum threshold of the population that is susceptible to the virus, halting the exponential growth of the curve and preventing the rest of the healthy population from ever being infected. 
D. It is based on the SIR model, because the constant 15% rate reflects the exact balance between newly infected individuals and those who are removed (either immunized or dead). This balance creates a permanent loop of infection that sustains the disease in the population over the years.
E. None of the above.

Original idea by: Yuri S. Costa

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