*The basic
reproductive rate (symbol R*_{0}) has proven a very useful (and comprehensible)
measure of the transmissibility or spreading potential of an infection in a population.It
is R_{0} which is actually illustrated in slide 5 (where R_{0}=3).

R_{0} 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).

R_{0} is independent of the number (or proportion) of individuals susceptible
or immune in a given population.

R_{0} 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. R_{0}
is, therefore, specific to a microorganism and a population.

__MORE: Estimated values of R___{0} for different infections
and populations.