The Pathogen Population

Pathogens are microbial agents that cause disease in animal and plant hosts and in other microbes. In the context of invertebrate animal hosts, the pathogens include viruses, entomopathogenic nematodes (EPNs), bacteria, fungi, microsporidia. Invasion of a potential host is necessary for infection but does not necessarily mean that an infection will occur. Before reproduction can begin, the pathogen must invade the host, overcome the host's immune defenses and access the appropriate tissues for reproduction, a species-specific characteristic known as tissue tropism. Pathogenicity and virulence depend on interactions with a specific host, pathogens that produce different responses may or may not be different strains. The survival of pathogens and pathogen populations requires that they interact, directly or indirectly, with other biotic factors and with abiotic factors in the environment. Vertically transmitted pathogens usually express low virulence and cause chronic or covert infections. In some host-pathogen interactions, vertical transmission is part of a more complex lifecycle.