As mentioned, disease progression within a host organism is a complex and dynamic process. There is a multitude of factors both about the pathogen and the host which affect bacterial pathogenicity and thus cause any number of different outcomes for a previously healthy organism.
Bacterial Infectivity
It is in the best interests of the pathogen not to kill the host as its objective is to multiply. However, if there is an imbalance between host resistance and bacterial virulence, this may lead to increased pathogenicity and potentially fatal outcomes.
Host Susceptibility
A healthy, intact immune system, including phagocytic cells, is vital for the best possible defense against pathogenic infections. Initially, resistance is conferred by non-specific mechanisms, with specific immunity developing over time. Any number of factors can lead to a compromised immune system and therefore increased host susceptibility.
Factors that can affect host susceptibility include conditions such as HIV which cause immunosuppression, aging (older people are more susceptible to disease than younger patients) medication and treatment for conditions such as cancer, and in some cases diet and nutrition and the health of the host’s microbiome.
Host Resistance
Host resistance plays a large part in the severity of infection progression. Numerous attributes of the host come into play when the host is exposed to pathogens, both chemical and physical. These include mucosal surface secretions with several antibacterial factors and phagocytic cells which mediate inflammatory responses through complex signaling pathways.
Many lymphoid cells participate in this inflammatory response. To be able to effectively infect an organism, pathogenic microorganisms must be able to overcome these defense mechanisms and host attributes.