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Transmission of an infection requires three conditions:
An effective contact is defined as any kind of contact between two individuals such that, if one individual is infectious and the other susceptible, then the first individual infects the second. Whether or not a particular kind of contact will be effective depends on the infectious agent and its route of transmission.
The effective contact rate in a given population for a given infectious disease is measured in effective contacts per unit time. This may be expressed as the total contact rate , multiplied by the risk of infection, given contact between an infectious and a susceptible individual. This risk is called the transmission risk and is denoted p. Thus:
The total contact rate, γ {\displaystyle \gamma } , will generally be greater than the effective contact rate, β, since not all contacts result in infection. That is to say, p can never be greater than 1, since it is effectively the probability of transmission occurring.