  Epidemic models are a valuable tool in the decision making process. Once a
mathematical model for an epidemics has been established, the very next step is
calculating a mathematical expression for the basic reproductive number, $R_0$,
which is the average number of infections caused by an individual that is
introduced in a population of susceptibles. Finding a mathematical expression
for $R_0$ is important because it allows to analyze the effect of the different
parameters in the model on $R_0$ so that we can act on them to keep $R_0 < 1$,
so that the epidemic fades out. In this work we show how to calculate $R_0$ in
complicated epidemic models by using only basic concepts of Markov chains.
