Population biology of multihost pathogens

The majority of pathogens, including many of medical and veterinary importa nce, can infect more than one species of host. Population biology has yet t o explain why perceived evolutionary advantages of pathogen specialization are, in practice, outweighed by those of generalization. Factors that predi spose pathogens to generalism include high Levels of genetic diversity and abundant opportunities for cross-species transmission, and the taxonomic di stributions of generalists and specialists appear to reflect these factors. Generalism also has consequences for the evolution of virulence and for pa thogen epidemiology, making both much Less predictable. The evolutionary ad vantages and disadvantages of generalism are so finely balanced that even c losely related pathogens can have very different host range sizes.