Pathogens are a broad group that include: (Infectious Disease is the specific branch of medicine that focuses on these infections and pathogens.) - Parasites - Bacteria - Viruses - Prions - Viroids - Fungi

All organisms with multiple cells(multicellular) are colonized to some extent by an extrinsic organisms. The large majority of these exist in either a relationship with the host like

Several variables are involved which determine the intruders outcome on the host: - The Route of entry - The access to host regions that it gains - The amount or load of the initial inoculant - The intrinsic virulence of the particular organism - The status of the immune system of the host that is being colonized
About the Author

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Ryan Fyfe is the owner and operator of Infection Area - http://www.infection-area.com, which is the best site on the internet for all infection related information.