Host–Symbiont Relationships: Understanding the Change from Guest to Pest

The several meanings for the term “symbiosis” create confusion, which can be avoided when the author provides details of the interrelationships between the symbiotic organism and the “host” so that a reader can clearly understand what definition is implied in each case. For example, we, as opposed to many other mentioned readers, consider a symbiont as an organism living in an association with another regardless of whether it causes a pathologic response or not, but from our title, the reader may incorrectly infer that we consider a parasite to be different from a symbiont. A symbiont is an organism that uses another organism as a habitat. This chapter discusses the primary associations and associated conflicts involving the terminology. It also provides both differentiation between and conflicting views regarding the interpretation of the terms “infect” and “infest,” “infection” and “disease,” and other terms. Many seemingly harmless symbionts of a wide array of taxonomic groups are triggered to become pathogenic or virulent, and we provide several examples of the provoking (stimulating) triggers, with the understanding that in most cases, the conditions for the triggered activities are much more complex and complicated than presented. Examples of triggers follow: environmental ones like temperature, toxic chemicals (dose), chemotherapeutics, dietary changes, and geographic habits; internal ones like host site, host resistance or susceptibility, and host modifications; and combinations of these and other conditions. We provide examples involving multiple triggers for organisms associated with termites, for an endemic virus being affected by multiple factors and having multiple effects on its commercial penaeid shrimp hosts, and for contrasting variables associated with two exotic viruses in wild and cultured commercial penaeid shrimps with an emphasis on hypothesizing how the pathogenicity developed in these two viruses. The chapter ends by trying to answer the question of why would a symbiont become pathogenic in some hosts and not in others from an evolutionary perspective. It uses two hypotheses to explain the increased virulence.