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

Epidemiological Models
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
1. New cases are usually recognized at a later clinical stage. 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.
Pre-pathogenesis period
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. This stimulus may cause actual signs and symptoms which present clinically, or may react with the host without symptoms as in the latency period.
NhoD: measures applicable to pre-pathogenesis and pathogenesis stages.
Classified as primary, secondary, or tertiary.

ex: Immunizations, health education on nutrition, exercise, the relationship of risk factors and avoidance of such (smoking cessation groups, prenatal classes)

Screenings (early detection)
Early pathogensis

ex: glaucoma, glucose, blood pressure, cholesterol, and developmental screenings

Treatment (prevent progression or complications)
Convalescence period

ex: traditional rehabilitation services and interventions for cardiac and stroke rehabilitation; teaching the newly diagnosed/discharged diabetic individual how to give insulin and identify signs and symptoms of hypo or hyperglycemia; halfway houses; sheltered workshops.