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=Introduction=

=Introduction=

[[Image:Routes of Infection.jpg|thumb|right|150px|Routes of Infection - B. Catchpole, RVC 2008]]

[[Image:Routes of Infection.jpg|thumb|right|150px|Routes of Infection - B. Catchpole, RVC 2008]]

Infection is a primary cause of disease.  Four pathways of infection are identified: bacterial, viral, fungi, and parasitic, comprising protozoa and worms (helminths).  In all cases, microorganisms can display similar characteristics.  They can manifest as '''primary (obligate) pathogens''' or '''secondary (opportunistic) pathogens''', the former of which always corresponds to disease, and the latter of which depends on prerequisites being fulfilled before causing disease.  As such, secondary pathogens can be found in healthy animals, but they lie in wait until a primary cause compromises the immune system and then manifest symptoms.  Secondary pathogens can even masquerade as '''commensal''' organisms, or those which operate in harmony with the animal, until they become pathogenic.

Infection is a primary cause of disease.  Four pathways of infection are identified: bacterial, viral, fungal, and parasitic, comprising protozoa and worms (helminths).  In all cases, microorganisms can display similar characteristics.  They can manifest as '''primary (obligate) pathogens''' or '''secondary (opportunistic) pathogens''', the former of which always corresponds to disease, and the latter of which depends on prerequisites being fulfilled before causing disease.  As such, secondary pathogens can be found in healthy animals, but they lie in wait until a primary cause compromises the immune system and then manifest symptoms.  Secondary pathogens can even masquerade as '''commensal''' organisms, or those which operate in harmony with the animal, until they become pathogenic.

Microorganisms can enter the body by three routes: '''contact''' (typically dermatological infection), '''aerosol''' (typically respiratory infection), or '''orofecal''' (typically enteric infection).  Infection can be either '''exogenous''' (via entry to epithelial surfaces) or '''endogenous''' (intracellular, extracellular, or vesicular).

Microorganisms can enter the body by three routes: '''contact''' (typically dermatological infection), '''aerosol''' (typically respiratory infection), or '''orofecal''' (typically enteric infection).  Infection can be either '''exogenous''' (via entry to epithelial surfaces) or '''endogenous''' (intracellular, extracellular, or vesicular).