Force of Infection

In a communicable disease, the number of new cases occurring in a period of time is dependent on the number of infectious persons within a susceptible population and the degree of contact between them. Persons, whether infectious or susceptible, and a period of time are all quantifiable factors, but the degree of contact can depend upon very many variables (some of which have been covered above). The factors, such as proximity (density) of populations, carriers, reservoirs, climate and seasonality, will have separate effects. To single these out and ascribe values to them will involve considerable, and generally unnecessary, complexity. In some disease patterns, certain factors have sufficient influence that they require to be given values, but for the time being, it is best to consider these altogether as a force of infection. This can be summarized as:

The force of infection = Number of infectious individuals x Transmission rate

Therefore:

Number of newly infected individuals = Force of infection x Number of susceptible individuals in the population

If the susceptible population is sufficiently large to maintain a permanent pool of susceptibles (as would happen in a disease where there is little or no immunity) and the force of infection is constant, then newly infected individuals will continue to be produced, while infectious individuals remain in the population. One healthy

© R. Webber 2005. Communicable Disease Epidemiology and Control, 2nd edition (Roger Webber)

carrier might continue to infect a large number of individuals over a long period of time, or a brief devastating epidemic, with a short period of infectiousness, may infect a large number of people over a short period of time. Parasitic infections, such as hookworm, would be an example of the former and measles, an example of the latter. Of course, measles produces immunity, which will alter the size of the susceptible population.

The proportion of susceptible individuals can be reduced by mortality, immunity or emigration, or increased by birth or immigration. After a certain period of time, a sufficient number of non-immune persons would have entered the population for a new epidemic of the disease to occur.

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