Are there pros as well as cons to being parasitized?

The diversity of ways in which parasites reduce the fitness of their hosts has been documented during the past decades, and clearly indicates that par asites can often be considered as direct agents of selection. In natural sy stems, however, the outcome of a host-parasite interaction might be strongl y determined by other ecological factors. Parasites can be detrimental to h ost fitness in one environment, whereas they can be beneficial to if in ano ther. From an evolutionary perspective, this phenomenon is of considerable importance for understanding the dynamics of coevolution among geographical ly structured populations evolving under different ecological pressures. He re, Frederic Thomas and colleagues review several ecological situations in which parasitized individuals enjoy a selective advantage over unparasitize d conspecifics.