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DEFINITIONS in Medical research & Epidemiology


 Click on this link to download the entire 5th edition, Dictionary of Epidemiology:  


AETIOLOGY / ETIOLOGY:   The study of all the mechanisms and determinant of the disease, which led to the illness.

AGENT:  Any microbiological, chemical, or physical exposure or substance which can give rise to a disease.    


CASE FATALITY RATE  The risk of dying given that you are a case.   This is the number of deaths from a disease over the number of cases

CARRIER: A person or animal that harbours a specific infectious agent in the absence of discernable clinical disease and serves as a potential source of infection. The carrier state may exist in an individual with an infection that is inapparent throughout its course (healthy or asymptomatic carrier), or during the incubation period, convalescence, and post-convalescence of an individual with a clinically recognizable disease (incubatory or convalescent carrier). Under either circumstance, the carrier state may be of short or long duration (temporary or transient carrier, or chronic carrier)

CLINICAL:  Showing symptoms (Comes from the old Greek word for "bed", thus these were the signs that a physician could see and record while attending the bedside of the patient).

CONTACT:  A person or an animal: that has been in an association with an infected person or animal or a contaminated environment that might provide an opportunity to acquire the infective agent.


DISINFECTANT:  Killing of infectious agents outside the body by direct exposure to chemical or physical agents.  D is only necessary for diseases spread by direct contact.

Concurrent disinfection:  is the application of disinfective measures as soon as possible after the discharge of infective material from the body of an infected, or after the soiling of articles with such infectious discharges; all personal contact with such discharges or articles should be minimized prior to such disinfection.

Terminal disinfection : is the application of disinfective measures after the patient has been removed by death or to a hospital, or has ceased to be a source of infection, or after hospital isolation or other practices have been discontinued. TD is rarely practiced.  T cleaning is usually sufficient.  Steam sterilization or incineration of bedding and other items is recommended after a disease such as Lassa fever or other highly infectious diseases.

ENDEMIC:  The constant presence of a disease or infectious agent within a given geographic area.  May also refer to the usual prevalence of a disease within such area.

ENZOOTIC  An "Endemic" in a non-human species ]

EPIDEMIC: The occurrence un a community or region of cases of an illness (or an outbreak) clearly in excess of expectancy.  The number of cases indicating presence of an epidemic will vary according to the infectious agent, size and type of the population exposed, previous experience or lack of exposure to the disease, and time and place of the occurrence; epidemicity is thus relative to the usual situation; frequency of the disease in the same area, among the specified population at the same season of the year. Usually TWO cases of a communicable disease not previously seen or long absent from the community can be considered an epidemic.  The equivalent response to an dangerous rare disease such as Botulism can also be considered an outbreak or epidemic.

EPIDEMIOLOGY  See discussion about definitions in Chapter 1 of the notes.

EPIZO÷TIC  An "Epidemic" in a non-human species

FOMITE:   an article which can convey infection from one person to another- usually as an accidental process, and not as the main source of the infection.  Such articles might include clothing, combs, towels, newspaper, bedding, eating utensils, paper handkerchief, toothbrush. 



HORIZONTAL TRANSMISSION:  Transmission of a disease between individuals through contact or other environmental routes such as animals in the same pasture, people in a family, or children in a day-care.

HOST: A person or other living animal, including birds and arthropods, that affords subsistence or lodgement to an infectious agent under natural (as opposed to experimental) conditions.  Some protozoa and helminths pass successive stages in alternative hosts of different species.

IATROGENIC:  Arising from treatment itself. Thus an Iatrogenic infection is an infection acquired during the course of being treated.

ICEBERG model of disease presentation

INCIDENCE: Any measure of new cases appearing in that community

INCIDENCE RATE: A rate with the number of new cases of a specific disease diagnosed or reported during a defined period of time as the numerator, and the number of persons in the studied population as the denominator.  This is normally expressed as cases per 100, per 1,000, per 10,000, etc.,  per year.  May also be expressed as sex-specific or age-specific, etc.  [see course notes for more details of rates used]   

INCUBATION PERIOD: The time interval between the initial contact with an infectious agent and the appearance of the first sign or symptom of the disease in question, (or in a vector, of the first time transmission is possible).


ISOLATION : Separation, for the period of communicability, of infected persons or animals from others in such places and under such conditions as to prevent or limit the direct transmission of the infectious agent from those who are susceptible or who may spread the agent to others.   [See CCDM for specific types of Isolation ]

LATENT DISEASE     Disease that is "hidden" or delayed but that may become clinical/symptomatic at some future point

LESION Any physical sign or symptom such as a rash, swelling, haemorrhage, 

MORTALITY RATE: [See course notes for types of rates]

NON-CLINICAL    Without showing symptoms

NOSOCOMIAL INFECTION     Infection inadvertently acquired at the place of treatment (e.g. clinic or hospital)

PANDEMIC:   Epidemic which involves several regions or countries (See also CCDM definition)

PANZO÷TIC:  Equivalent to PANDEMIC but in non-human species

PATHOGEN: an infectious agent that can cause disease

POINT SOURCE:  a single point in time and place where the exposure to a disease took place, as opposed to simple person-to-person spread.  Of course an initial exposure point (primary cases) may then be followed by propagated (person-to-person) spread as the primary cases transmit the disease to a secondary "wave".  Qualifications and variations may include multiple- or repeat exposures to the point source over time 

PRECLINICAL STAGE OF DISEASE - .The stage immediately before the clinical symptoms begin.  Not all diseases have a recognizable pre-clinical stage.  But in those that do, it means the time when early changes are taking place in the immune response system, that are detectable through lab (e.g. blood) tests but not yet showing as a clearly recognizable symptom.  Note that while in a pre-clinical stage, the patient is also said to be "sub-clinical"   

PREVALENCE RATE: A rate where the numerator is the number of ill persons or those showing a specified sign or condition in a stated population at a particular time (point prevalence) or during a stated period of time (period prevalence) regardless when the illness or condition began.  The denominator in both rates is the number of persons in the population under study.  

PRODROME    Immediately before obvious symptoms appear, patient may be infectious or have subtle changes in condition   

PROPAGATION     Process by which the disease is transmitted from person to person

QUARANTINE: Restriction of the activities of well persons or animals (i.e. contacts) who have been exposed to a case of communicable disease during its period of communicability, to prevent disease transmission during the incubation period if infection should occur.  [See CCDM for specific types of Q]

RESERVOIR: Any person, animal, arthropod, plant, soil or substance (or combination of these) in which an infectious agent normally lives and multiplies, on which it depends primarily for survival, and where it reproduces itself in such manner that is can be transmitted to a susceptible host.

STERILIZE:   To expose the subject material to a process or or chemical sufficient to render it devoid of all viable microbial life forms.   Note that "commercially sterile" denotes a very high level of sterility (typically at least a 6 or 7 log reduction) but this may not be absolutely sterile.  

SUB-CLINICAL: Where the changes as part of the disease process are 'below' the level where they can be recognized as a clear set of symptoms. Note that PRE-CLINICAL changes (see above) are usually SUB-CLINICAL for this same reason.  But whereas the pre-clinical stage will usually soon progress to the clinical stage, with the usual signs and symptoms, the sub-clinical condition may remain "invisible" or may progress at some future time. An example of SUB-CLINICAL would include those cases of human infection with H5N1 influenza who do not show any signs of the disease. (Unfortunately these are very rare so far).  Latent tuberculosis is also said to be sub-clinical.        


SYNDROME     The grouping of symptoms characteristic of a particular disease

TRANSMISSION: [see CCDM for types and details]

TRIAD model of relationships between disease factors

VECTOR     Living creature that forms part of the chain of infection; e.g.: anopheline mosquito in malaria

VEHICLE     (Non-living) link in chain of infection, substance by which the infection is spread; e.g. raw milk in brucellosis

VERTICAL TRANSMISSION   Disease transmission directly from parent to offspring through normal gestational processes, or any of the fluids or processes involved in reproduction (semen, milk, placenta, vaginal fluids, etc).  Thus all genetic diseases are vertically transmitted, and among infectious diseases, we can include congenital syphilis (passed to the developing fetus by the infective mother).    This would NOT include accidental infection of a calf by a cow as a result of contamination, of a type where such contamination may have taken place between any two animals.


XENOTRANSPLANTATION     Process of using other species' organs for transplanting into humans

ZO÷NOSIS    Disease that can be transmitted from animals to humans; e.g.: Rabies, bovine tuberculosis, undulant fever.  Includes a growing list of "newer" diseases that have origins in animal reservoirs (Ebola f., SARS, Hanta f., Nippah f., BSE/vCJD, etc.)