TABLE 1.

Some historical definitions in the field of microbial pathogenesis^a

Term	Definitionb	Reference
Carrier state	The retained invader (e.g., microbe)—under the influence of the immune environment—gradually dissociates into a saprophytic state	57
	A state of animal adaptation whereby the microbe and its products cause no damage; in this state the “organisms themselves have probably developed a state of resistance against substances which ordinarily would destroy the organisms and neutralize their products”	25
	When an organism of relatively high pathogenicity may appear in the normal flora without causing disease	44
	Certain individuals may continue to harbor a pathogen after clinical recovery from an infectious disease and may serve as carriers of infection	35
Colonization	An agent is considered to colonize a host when its presence in that host does not cause a specific immune response or infection	34
	Microorganisms which do not belong to the normal flora of the host but do not inflict local damage to the host	51
	The appearance or increase in numbers of a particular invasive bacterial species in the resident microflora	52
	Implantation of a microbe at a site, such as multiplication of staphylococci in the anterior nares	24
	Multiplication of an organism on a body surface without evoking an immune response	13
Commensal	A harmless parasite	36
	The organisms of the normal flora	44
	Microbes that can establish themselves in the throat, nose, or intestines without damage to the host	53
	Nonpathogenic organisms present in varying numbers at sites of the normal host's body that are in contact with the environment	1
	An organism which “eats at the same table” as another of a different species but which confers on the latter neither benefit nor harm	35
	Pertaining to or characterized by commensalism; an organism participating in commensalism	28
Commensalism	The mutual but almost inconsequential association between bacteria and higher organisms	57
	The presence of microorganisms on skin and mucous membranes	33
	A symbiotic association between host and microorganism in which the microorganism is benefitted but the host is neither helped nor harmed	32
	An organism that lives in close association with another of a different species without either harming or benefitting it	23
	A form of parasitism in which no injury is dealt to either participant by the other	19
	The ability (of a microorganism) to live on the external or internal surfaces of the body without causing disease	48
	A symbiotic relationship in which one species derives benefit and the other is unharmed	28
Germ carrier or carrier	A person who harbors and releases pathogenic organisms without manifesting symptoms of the disease associated with the pathogen	24
	A host that harbors a pathogenic organism in a commensal state	36
	Healthy individuals who harbor in their body parasitic organisms which are harmful to others	18
	Referred to as subinfection, the state whereby a microbe is intimately associated with and has its normal habitat in a certain part of the body and does no harm until special conditions arise, when it may rapidly invade the tissues and produce infection	26
	A carrier is a person, animal, or arthropod who harbors a specific infectious agent in the absence of clinical illness with or without a detectable immune response	13
Infection	The invasion of the body tissues by microorganisms resulting in disease	18
	When microparasites have passed the normal barriers of the skin or mucous membranes and have invaded and proliferated in the deeper tissues	26
	A process in which an organism enters, establishes itself, and multiplies in the host (not in others)	32
	Invasion of the body by harmful organisms (pathogens), such as bacteria, fungi, protozoa, rickettsiae, or viruses	23
	The process whereby pathogenic organisms become established and multiply in or on the body of the host	24
	The deposition, colonization, and multiplication of a microorganism in a host; usually accompanied by a host response	13
	Invasion of the body with organisms that have the potential to cause disease	28
Infestation	Distinct from infection in that it applies specifically to animal parasites of macroscopic size, such as intestinal worms	26
Infectious disease	The result of parasitism in which no mutual adaptation has taken place and in which the invasion of the host by the parasite is marked by a struggle, the local and systemic manifestations of which constitute the disease	56
	The abnormal state resulting from the deleterious local and general interaction between a host and an invading parasite, with consequent tissue changes and symptoms	26
	The manifestations of the fight between the disease-producing or pathogenic organisms and the host with all its defense mechanisms	33
	Infection that becomes apparent	32
Mutualism	A relation between two dissimilar organisms in which both are benefitted	32
	Commensalism in which the relationship is mutually beneficial	19
Opportunist or opportunistic	Microbes which cause no overt clinical or pathological conditions in the normal state but can become invasive when the defenses are disturbed	37
	Pathogens which attack persons with compromised immune function	35
	These infections represent the colonization of normally sterile tissues by bacteria from tissues that always support autochthonous populations, and because these autochthonous organisms are well adapted for survival on other tissue surfaces of the same animal, their control and clearance poses a whole spectrum of unique problems	8
	Pathogen not able to cause disease in healthy hosts but only in those with impaired defense mechanisms	1
	Normally harmless organisms which take the opportunity afforded by lowered host resistance to act as pathogens	48

^aNot a complete list. Definitions are representative of the variable definitions used for these terms encountered in the literature. 

^bIn most cases the definition was taken verbatim from the source stated. However, the wording of some sources was modified to construct a definition based on the meaning implied as understood by the authors.