Epidemiology

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Epidemiological Terminologies

Epidemiology investigates the distribution of diseases, their physiological variables and social consequences in human populations, and the factors that influence disease distribution (World Health Organization [WHO] definition). The field covered by this discipline can thus be defined as medical problems involving large collectives. The rule of thumb on infectious diseases is that their characteristic spread depends on the virulence of the pathogen involved, the susceptibility of the threatened host species population, and environmental factors.

Transmission, Sources of Infection

Transmission

Pathogens can be transmitted from a source of infection by direct contact or
indirectly. Person-to-person transmission constitutes a homologous chain of infection. The infections involved are called anthroponoses. In cases in which the pathogen is transmitted to humans from other vertebrates (and occasionally the other way around) we have a heterologous chain of infection and the infections are known as zoonoses (WHO definition)

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Transmission pathways of pathogenic organisms

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Examples of zoonoses caused by viruses and bacterias.

TERMINOLOGIES

Sporadic occurrence

Isolated occurrence of an infectious disease with no apparent connections between localities or times of occurrence.

Endemic occurrence

Regular and continuing occurrence of infectious diseases in populations with no time limit.

Epidemic occurrence

Significantly increased occurrence of an infectious disease within given localities and time periods

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Pandemic occurrence

Significantly increased occurrence of an infectious disease within a given time period but without restriction to given
localities.

Morbidity

Number of cases of a disease within a given population (e.g., per 1000, 10 000 or 100 000 inhabitants)

Incidence

Number of new cases of a disease within a given time period

Prevalence

Number of cases of a disease at a given point in time (sampling date)

Mortality

Number of deaths due to a disease within a given population

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Lethality

Number of deaths due to a disease in relation to total number of cases of the disease.

Manifestation index

Number of manifest cases of a disease in relation to number of infections

Incubation period

Time from infection until occurrence of initial disease symptoms

Prepatency

Time between infection and first appearance of products of sexual reproduction of the pathogen (e.g., worm eggs in stool)

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