Pathogen Terminologies


Infectiological Terminology I (Pathogen)

Term and Explanation


These microorganisms are nonpathogenic; their natural habitat is dead organic matter.


Unicellular or metazoan organism living in or on an organism of another species (host) on the expense of the host


Normal inhabitants of skin and mucosa; the normal flora is thus the total commensal population

Pathogenic microorganisms

Classic disease-causing pathogens

Opportunists or facultatively pathogenic microorganisms

Can cause disease in immunocompromised individuals given an “opportune” situation; these are frequently germs of the normal flora or occasionally from the surrounding environment, animals, or other germ carriers


Capacity of a pathogen species to cause disease.


Sum of the disease-causing properties of a strain of a pathogenic species


periodTime between infection and manifestation of disease symptoms; this specific disease characteristic can be measured in hours, days, weeks,
or even years

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A parasitological term: time between infection and first appearance of products of sexual reproduction of the pathogen (e.g., worm eggs in stool of a host with helminthosis)

Infection spectrum

The totality of host species “susceptible” to infection by a given pathogen

Minimum infective dose

Smallest number of pathogens sufficient to
cause an infection

Mode of infection

Method or pathway used by pathogen to invade.


Microbiological presence of microorganisms on objects, in the environment, or in samples for analysis


Presence of microorganisms on skin or mucosa; no penetration into tissues; typical of normal flora; pathogenic microorganisms occasionally also show colonization behavior


Invasion of a host organism by microorganisms, proliferation of the invading organisms, and host reaction

Inapparent (or sub-clinical) infection

Infection without outbreak of clinical symptoms.

Infectious disease
(or clinical infection)


Infection with outbreak of clinical symptoms

Probability of

Frequency of clinical manifestation of an infection in disposed individuals

Endogenous infection

Infection arising from the colonizing flora

Exogenous infection

Infection arising from invasion of host by microorganisms from sources external to it

Nosocomial infection

Infection acquired during hospitalization (urinary tract infections, infections of the respiratory organs, wound infection,

Local infection

Infection that remains restricted to the portal of entry and surrounding area

Generalized infection

Lymphogenous and/or hematogenous spread of invading pathogen starting from the portal of entry; infection of organs to which pathogen shows a specific affinity (organo-tropism); three stages: incubation, generalization, organ manifestation


Systemic disease caused by microorganisms and/or their toxic products; there is often a localized focus of infection from which pathogens or toxic products enter the bloodstream continuously or in intermittent phases

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Transitory bacteremia/

Brief presence of microorganisms in the bloodstream


Occurrence of a second infection in the course of a first infection


Series of infections by the same pathogen


Series of infections by different pathogens

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