Lec3: Epidemiological of Disease
3-1 Epidemiology of communicable disease:
Definition of communicable diseases:
A communicable disease is an illness due to a specific infectious (biological) agent or its toxic
products capable of being directly or indirectly transmitted from man to man, from animal to
man, from animal to animal, or from the environment (through air, water, food, etc..) to man.
Importance of Studying Communicable Diseases Epidemiology:
 Changes of the pattern of infectious diseases
 Discovery of new infections
 The possibility that some chronic diseases have an infective origin.
Infection:
 Infection is the entry and development or multiplication of an infectious agent in the body
of man or animals. An infection does not always cause illness.
 There are several levels of infection (Gradients of infection):
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Colonization (S. aureus in skin and normal nasopharynx)
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Subclinical or inapparent infection (polio)
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Latent infection (virus of herpes simplex)
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Manifest or clinical infection
Contamination:
 The presence of an infectious agent on a body surface, on or in clothes, beddings, toys,
surgical instruments or dressings, or other articles or substances including water and food
Epidemic:
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 “The unusual occurrence in a community of disease, specific health related behavior, or
other health related events clearly in excess of expected occurrence”
 Endemic: It refers to the constant presence of a disease or infectious agent within a given
geographic area or population group. It is the usual or expected frequency of disease
within a population.
 (En = in; demos = people)
Eradication and Elimination:
 Termination of all transmission of infection by the extermination of the infectious agent
through surveillance and containment. Eradication is an absolute process, an “all or none”
phenomenon, restricted to termination of infection from the whole world.
 The term elimination is sometimes used to describe eradication of a disease from a large
geographic region. Disease which are amenable to elimination in the meantime are polio,
measles and diphtheria.
Source or Reservoir:
 The starting point for the occurrence of a communicable disease is the existence of a
reservoir or source of infection.
 The source of infection is defined as “the person, animal, object or substance from which
an infectious agent passes or is disseminated to the host (immediate source). The
reservoir is “any person, animal, arthropod, plant, soil, or substance, or a 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 a manner that it can be
transmitted to a susceptible host. It is the natural habitat of the infectious agent.”
Carriers:
 It occurs either due to inadequate treatment or immune response, the disease agent is not
completely eliminated, leading to a carrier state.
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 It is “an infected person or animal that harbors a specific infectious agent in the absence
of discernible (visible) clinical disease and serves as a potential source of infection to
others.
 Three elements have to occur to form a carrier state:
1. The presence in the body of the disease agent.
2. The absence of recognizable symptoms and signs of disease.
3. The shedding of disease agent in the discharge or excretions.
Modes of transmission:
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3-2 Epidemiology of Non-Communicable Diseases
Chronic diseases definitions:
An impairment of bodily structure or function that necessitates a modification of the patient’s
normal life, and has persisted over an extended period of time
 Diseases comprising all impairments or deviations from normal, which have one or more
of the following characteristics:
a. Are permanent
b. Leave residual disability
c. Are caused by non-reversible pathological alterations
d. Require special training of the patient for rehabilitation
e. May be expected to require a long period of supervision, observation or care
Developing countries are now warned to take appropriate steps to avoid the epidemics of NCD
likely to come with socioeconomic and health development.
Causes:
 Rise in life expectancy and increasing number of senior citizens.
 Changing lifestyles: faulty diet, alcohol intake, sedentary life, obesity, stress and
Tobacco.
 Exposure to environmental risk factors- air pollution.
 Increasing population.
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Gaps in the natural history of NCD:
1. Absence of known agent: in most of NCD the cause is not known.
2. Multifactorial causation: in absence of causative agents, risk factors are studied
 An attribute or exposure that is significantly associated with development of disease.
 If determinant is modified by intervention, it reduces possibility of occurrence of
disease.
 Risk factors can be causative, contributory or predictive.
 They can be modifiable or non-modifiable
 They can be individual or community risk factors
 Epidemiological studies are needed to identify risk factors
 At-risk approach, at-risk groups, risk factors with diseases
3. Long latent period: it is the period between the first exposure to suspected cause and the
eventual development of disease. This makes it difficult to link suspected causes with
outcomes.
4. Indefinite onset: Most (NCD) are slow in onset and development. Distinction between
diseased and non-diseased may be difficult to establish.
Prevention of NCD:
Levels of prevention
1. Primordial
2. Primary
3. Secondary
4. Tertiary
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Primordial prevention- Prevention of the emergence or development of risk factors in
countries or population groups in which they have not yet appeared. Efforts are directed
towards discouraging children from adopting harmful life styles.
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Primary prevention- Action taken prior to the onset of disease which removes the
possibility that the disease will ever occur. Can be divided into population & high risk
strategy.
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Interventions:
 Health promotion
 Specific protection
 Adequate nutrition
 Safe water and sanitation
3. Secondary prevention-Action which halts the progress of the disease at its incipient stage
and prevents complications. Mostly curative. Disadvantage - patient has already suffered
mental & physical anguish & community to loss of production. Often more expensive
&less effective. Intervention – EARLY DIAGNOSIS AND TREATMENT
4. Tertiary prevention- defined as all measures available to reduce impairments &
disabilities, minimize suffering due to departure from good health & promote patient’s
adjustment to irremediable conditions .Intervention – DISABILITY LIMITATION AND
REHABILITATION
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