The basic reproductive rate (symbol R₀) has proven a very useful (and comprehensible) measure of the transmissibility or spreading potential of an infection in a population.

It is R₀ which is actually illustrated in slide 5 (where R₀=3).

R₀ is an abstraction: no population actually experiences spread of an infection at this rate (with the exception of the extreme situation of an isolated area, totally virgin to a microorganism, where an infectious case enters).

R₀ is independent of the number (or proportion) of individuals susceptible or immune in a given population.

R₀ is determined by basic biological features of the microorganism and of man as well as by social characteristics and public health practices in the population which relate to the risk of a potentially transmitting contact between individuals. R₀ is, therefore, specific to a microorganism and a population.

MORE: Estimated values of R₀ for different infections and populations.

(The definition of R₀ presented here is taken from: Giesecke J. Modern infectious disease epidemiology. London: Arnold, 1994.)