A MODERN VIEW OF THE EVOLUTION OF VIRULEN CE

According to the prevailing, traditional view parasites should develop reduced virulence towards their hosts, because more virulent pathogen s are more likely to drive the hosts, and thus themselves to extinctio n. Virulence is considered to be a primitive stage of a parasitive-hos t association. However the usefulness and validity of this view have b een questioned. Recent studies suggest that parasites need not necessa rily evolve towards reduced virulence. The points of view of Darwinian medicine in the direction of the evolution of virulence there may be many possible coevolutionary trajectories, depending on the details of the parasite's life-history the host's behavior and the transmissibil ity of the parasite. Theoretical and epidemiological evidences indicat e that pathogens transmitted by arthropod vectors are significantly mo re lethal to humans than those transmitted by personal contac. Water b orne enteric pathogens are less virulent after purification of water s upplies. Recent experiments also supper? the emerging theory that para sitism can evolve to be either more or less virulent in a long-term ho st, depending on the way the parasite is transmitted to the host and o n the environment in which they live.