From continuous standard SIR model, which is configured from two sequenced flows (a) susceptible – infectious and (b) infectious – removed, we obtain two impulsive SIR models assuming different time scales for (a) respect to (b) (one more quickly than the other and inversely). By associating respective stroboscopic maps to this impulsive systems, two discretizations are defined. The dynamics of these maps are analysed in order to get thresholds conditions for predicting (or to control) epidemic outbreaks. As it is traditional for SIR systems, we also find conditions for the final size of the susceptible group.