What is a pathogen? Toward a process view of host-parasite interactions

Abstract

Until quite recently and since the late 19(th) century, medical microbiology has been based on the assumption that some micro-organisms are pathogens and others are not. This binary view is now strongly criticized and is even becoming untenable. We first provide a historical overview of the changing nature of host-parasite interactions, in which we argue that large-scale sequencing not only shows that identifying the roots of pathogenesis is much more complicated than previously thought, but also forces us to reconsider what a pathogen is. To address the challenge of defining a pathogen in post-genomic science, we present and discuss recent results that embrace the microbial genetic diversity (both within- and between-host) and underline the relevance of microbial ecology and evolution. By analyzing and extending earlier work on the concept of pathogen, we propose pathogenicity (or virulence) should be viewed as a dynamical feature of an interaction between a host and microbes.

Keywords: disease; ecology; evolution; infection; process; virulence.

Fig. 1.

Spatial schema representing how the virulence of an infection arises (A) and how a biological association moves from virulence to pathogenicity (B). Note that in (A) the genotype only partly determines the phenotype and that the environment includes many factors (e.g., multiple infections). In (B), one needs to set a threshold value in order to decide when a parasite is virulent enough to be considered a pathogen. The rectangle illustrates the uncertainty in defining such a value.