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The probability of transmission during a contact with a susceptible and an infectious persson is p.

The probability of the susceptible person escaping infection during contact is

q (or 1 - p).

If a susceptible makes n contacts with an infective or with different infectives and each contact is independent of the others, the probability of escaping infection from all n potential infectives is qn = (1 - p)n .

The probability of being infected after n contacts is 1 - qn = 1 - (1 - p)n.