All of this occurs at the expense of the host. If the host and the intruder could coexist there would be no problem, but this isn't the case. The infecting organism in it's intrusion will interfer with the normal operation of the host, and in many cases inflict with the survival of the host. These organisms are usually consired to be tiny and microscopic, however the definition is in fact much wider than that.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 mentioned above. The difference between an is due to the circumstance which exists as a result of the former. Basically if the colonization of the external body comprises the life of the host organisms it is an infection.

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