As seen in the "SIRP diagram" of Figure 10 , a host begins as susceptible (S) in the upper left. They can become infected (I), and then they either recover (R) or are removed (removal not shown). After recovery (R), they have an immunity level that starts at or near complete as in SIR. This immunity declines, in the case of influenza, over a period of roughly 2 years, with a steep drop-off beginning after the first year, and hence is termed partial immunity(P). (It is not actually the case that the immune system's response to influenza antigens degrades so rapidly; rather, the virus mutates rapidly, and host immunity declines. The net result is similar.) This decline in immunity models the real world antigenic mutation of influ-enza viruses. In a more perfect model, the influenza virus would change and multiple viruses would be modeled. At some point the immunity declines so that it is effectively zero in the model (which corresponds to large antigenic change) and the person is returned to the fully susceptible(S) population. A person can become reinfected (diagonal arrow), though their probability of reinfection is lower, during this period of declining immunity.