I will start the presentation by briefly introducing the basics of the mathematical theory for epidemics in the simplest context of unstructured populations. I will then extend the same ideas to models of epidemic spread in populations consisting of different types of individuals and then to populations partitioned into households. Finally, I will add the presence of schools to obtain an even more complex model, for which investigation via simulation is mandatory, and I will use it to draw some policy recommendations on the optimal use of school closure as a form of intervention during an influenza pandemic.