When the body gets infected by a virus, the immune system develops a certain resistance against it. As a matter of fact disease-induced immunity tends to wane and, long time after recovery, an individual might become again susceptible to the virus. Exposure to the pathogen boosts the immune system, thus prolonging the time in which the individual is immune. Our aim is to model the feedback mechanism which makes possible for certain individuals to have lifelong immunity, being regularly exposed to the infection. In this talk we propose a new class of SIRS models in which the immune population structured by immune status. The general approach allows to consider the effects of waning immunity and immune system boosting on epidemics outbreaks. Moreover, under certain assumptions, it is possible to obtain well known ODE/DDE epidemic models from our system.