A Threshold Model with Temporary Immunity

Abstract

In the previous models it was assumed that contracting the disease provided permanent immunity and hence the class R of removed individuals was the final resting place for all individuals who became infective. With many diseases, however, the individual, after recovering from an infection, may become susceptible again. We now consider a model, described symbolically,

in which the individual in R is temporarily immune to the infection but eventually returns to the susceptible class. Although the description of the model differs from the previous one only on this point, the resulting model produces significantly more difficult mathematical problems.