Pathogens and parasites pose significant threats to zoological and botanical systems. The distinct bodies of research addressing pathogen evolution in plants and animals both seek to predict and control the extent to which pathogens manipulate hosts. ‘Virulence’ is a term used in both fields to describe a key component of pathogens’ strategies for accomplishing this. In the plant literature, ‘virulence’ is equivalent to infectivity, or the range of host (geno)types a pathogen is able to infect. In the animal and human literature, ‘virulence’ is equivalent to aggressiveness, or the rate of host damage (or alternatively the rate of reductions in fitness) inflicted by the pathogen. Separate frameworks have been proposed to explain the limits to the evolution of these two types of virulence. Is the separation of these frameworks appropriate? What insights can be gained by combining them?
Results/Conclusions
The two ‘virulence’ concepts employed by studies of plant and animal pathogens reflect differences in the coevolutionary processes that are typical of each system. These differences are largely the product of differences in immune defense systems, however, ecological factors likely are involved as well. Departures from the coevolutionary norms of each system bend or break the limits of the respective frameworks for pathogen adaptation. By considering the two ‘virulence’ concepts to be endpoints of a spectrum of pathogen strategies, and by creating a unified framework for limits to pathogen adaptation, we can better predict and manage pathogen evolution, and gain a deeper understanding of the coevolutionary processes that shape infectious disease systems.