Résumé
In this chapter, we review some of the classic theoretical results on virulence evolution in pathogens. Our goal is to contrast an intuitive vision of this question to results that can be obtained using simple modeling tools. The main conclusion of this chapter could be that, in pathogens as in many other natural systems, natural selection does not maximize the basic reproductive rate R-0. We illustrate this point with several scenarios that take into account several complexities such as superinfection and vertical transmission. We conclude this chapter with some considerations on how the use of these simple models can shed light on some experiments and, in return, how some experiment data is required to improve our theoretical predictions on pathogen evolution.