The role of parasitism in natural population has become re-considered in
the light of evolutionary thinking for barely more than two decades. A
priori principles have led to the formulation of new concepts that are
intensively studied in the rapidly increasing field of evolutionary
parasitology. In an insect-trypanosome model system, our lab has studied
some of those questions, including issues relating to the costs of host
defence or parasite strategies of diversification. New issues have recently
arisen such as, for example, the presence of additional players in the
host-parasite interaction.