Pathogenicity is the ability to produce disease in a host organism
Microbes express their pathogenicity by means of their virulence, a term which refers to the degree of pathogenicity of the microbe
The relationship between a host and a pathogen is dynamic, since each modifies the activities and functions of the other
The outcome of such a relationship depends on the virulence of the pathogen and the relative degree of resistance or susceptibility of the host, due mainly to the effectiveness of the host defense mechanisms
Hence, general requirements for any organism to cause disease within a host are: evasion of host defenses
, gaining access to the host via a portal of entry, adherence to host tissues
