Final answer:
In the S-I-R model, susceptible individuals decrease because they become infected, move to the infected group and potentially recover or die, leaving fewer individuals susceptible to the illness. This reduction is not due to the overall population decline but the natural progression of disease transmission and immunity development within the population.
Step-by-step explanation:
In a typical S-I-R model, which stands for Susceptible-Infected-Recovered, the number of susceptible people decreases over time because individuals contract the disease and move from the susceptible category to the infected category. This is not necessarily due to a declining total population, but rather to the dynamics of disease transmission and recovery in a population.
As individuals become infected, they leave the susceptible pool and enter the infected pool, hence the susceptible group decreases. The model assumes that the total population size remains roughly constant, with the changes in each compartment (S, I, and R) being the main focus. Once individuals recover or die from the disease, if they are conferred with immunity, they move to the recovered category or are removed from the model, which further reduces the susceptible population.
Pathogens might decline during the period of decline, leading to fewer signs and symptoms, but this coincides with a decrease in the number of susceptible individuals because most have either faced the infection or received immunity from it. Moreover, in certain scenarios, such as the Stage 3 and 4 of demographic transition, infection related decreases in susceptible individuals may also correlate with changes in birth and death rates influencing overall population dynamics.