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  We see here what would happen if some individuals in the population (say one in three) were already immune due to vaccination or previous natural exposure to the microorganism.

The spread of the infection is interrupted (black  lines in the slide) when the infectious cases contact (in a way that can potentially transmit the disease) an immune rather than a susceptible person. On average, this will happen in one in three contacts (as we assumed that 1/3 persons in the population are immune).

Therefore, a number of susceptibles will be indirectly protected from infection just because their neighbour is immune.