refers to the usual presence of a disease within a geographic area.
characterized by the occurrence of a disease clearly in excess of the normal expectancy within a geographic area; the disease is transmitted either from a single source or from sources propagated from a single source.
an epidemic affecting several countries or continents or much of the world.
Epidemiological triad in terms of cholera
agent (the unknown poison)
host (the person contracting cholera)
environment (the water transmitting the poison)
Etiologic factors related to the prepathogenesis period: BEINGS
B - Biological factors; Behavioral factors
E - Environmental factors
I - Immunological factors
N - Nutrition
G - Genetic
S - Services (health and social); Social factors; Spiritual factors
data collection includes
person (age groups, gender, occupation, etc)
place (geographic concentration)
purpose: determine patterns of incidence of disease and distribution of cases and to identify a source or sources of an epidemic
characterize the interactions between disease and communities.
examples: Natural History of Disease, Epidemiological Triad, Adaptive, Web of Causation
Natural History of Disease
Analyzes the occurrence of disease in a host over a period of time. This model divides the natural history of the disease into three distinct phases:
Phase 1: A new disease is identified.
Phase 2: Information is gathered on it.
Phase 3: Experimental studies are conducted.
NHoD: Phase 1
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. Clinicians in the community may be unable to diagnose a syndrome; they then either do nothing, report it to the CDC, or report it in the published literature. After there is an awareness of this new disease, additional case data will be collected. Before any formal epidemiological studies begin, epidemiologists must be sure that the syndrome is a "unique new disease" or a "unique complex of characteristics that together result in a specific pathological condition."
2. The next step is to begin case finding. This can be done by publishing preliminary information on the cases in state health publications or in the Morbidity and Mortality Weekly Report (MMWR).
NHoD: Phase 2
Information is gathered on the new disease via all reported cases: potential causal factors, incidence and prevalence, duration of stages of the illness, and potential risk factors associated with each stage.
Decisions are made about which hypotheses of the natural history of disease will be studied.
The occurrence of the disease, i.e., how many times it has occurred.
The number or proportion of cases in a given population.
NHoD: Phase 3
The experimental stage. The natural history of disease can only be conclusively determined by experimental evidence from formal experimental studies or randomized controlled trials.
The period before disease has infected the individual. During this time the individual is exposed to the agent(s) responsible for the disease.
Period of pathogenesis
Pathogenesis refers to the natural course of a disease after infection. In the diagram, agent, host, and environmental factors interact in such a way that there is an imbalance causing disease to occur
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