Epidemiologic Mind : Natural History of Disease Levels of Disease
Prevention
The “natural history of disease” refers to the progression of disease
process in an individual over time, in the absence of intervention. There
are four stages in the natural history of a disease.
These are:
1. Stage of susceptibility
2. Stage of pre-symptomatic (sub-clinical) disease
3. Stage of clinical disease
4. Stage of disability or death
1. Stage of susceptibility
In this stage, disease has not yet developed, but the groundwork has been
laid by the presence of factors that favor its occurrence.
Example:
unvaccinated child is susceptible to measles.
2. Stage of Pre-symptomatic (sub-clinical) disease
In this stage there are no manifestations of the disease but pathologic
changes (damages) have started to occur in the body. The disease can only
be detected through special tests since the signs and symptoms of the
disease are not present.
Examples:
• Detection of antibodies against HIV in an apparently healthy person.
• Ova of intestinal parasite in the stool of apparently healthy children.
The pre-symptomatic (sub-clinical) stage may lead to the clinical stage, or
may sometimes end in recovery without development of any signs or symptoms.
3. The Clinical stage
At this stage the person has developed signs and symptoms of the disease.
The clinical stage of different diseases differs in duration, severity and
outcome. The outcomes of this stage may be recovery, disability or death.
Examples:
-Common cold has a short and mild clinical stage and
almost everyone recovers quickly.
-Polio has a severe clinical stage and many patients develop
paralysis becoming disabled for the rest of their
lives.
-Rabies has a relatively short but severe clinical stage
and almost always results in death.
-Diabetes Mellitus has a relatively longer clinical stage and
eventually results in death if the patient is not
properly treated.
4. Stage of disability or death
Some diseases run their course and then resolve completely either
spontaneously or by treatment. In others the disease may result in a
residual defect, leaving the person disabled for a short or longer
duration. Still, other diseases will end in death.
Disability is limitation of a person's activities including his role as a
parent, wage earner, etc Examples:
• Trachoma may cause blindness
• Meningitis may result in blindness or deafness. Meningitis may also
result in death
Levels of Disease Prevention
The major purpose in investigating the epidemiology of diseases is to learn how to prevent and control them. Disease prevention means to interrupt or slow the progression of disease.
Epidemiology plays a central role in disease prevention by identifying those modifiable causes. There are three levels of prevention
1) Primary prevention:- The main objectives of primary prevention are promoting health, preventing exposure and preventing disease. Primary prevention keeps the disease process from becoming established by eliminating causes of disease or increasing resistance to disease. Primary prevention has 3 components. These are health promotion, prevention of exposure, and prevention of disease.
A. Health promotion: - consists of general non-specific interventions that enhance health and the body's ability to resist disease. Improvement of socioeconomic status, provision of adequate food, housing, clothing, and education are examples of health promotion.
B. Prevention of exposure: - is the avoidance of factors which may cause disease if an individual is exposed to them. Examples can be provision of safe and adequate water, proper excreta disposal, and vector control.
C. Prevention of disease :- is the prevention of disease development after the individual has become exposed to the disease causing factors. Immunization is an example of prevention of disease.
Immunization acts after exposure has taken place. Immunization does not prevent an infectious organism from invading the immunized host, but does prevent it from establishing an infection. If we take measles vaccine, itwill not prevent the virus from entering to the body but it prevents the development of infection/disease.
2) Secondary prevention:- The objective of secondary prevention is to stop or slow the progression of disease so as to prevent or limit permanent damage. Secondary prevention can be achieved through detecting people who already have the disease as early as possible and treat them. It is carried out before the person is permanently damaged.
Examples :
• Prevention of blindness from Trachoma
• Early detection and treatment of breast cancer to prevent its progression to the invasive stage, which is the severe form of the disease.
3) Tertiary prevention: – is targeted towards people with permanent damage or disability. Tertiary prevention is needed in some diseases because primary and secondary preventions have failed, and in others because primary and secondary prevention are not effective. It has two objectives:
• Treatment to prevent further disability or death and
• To limit the physical, psychological, social, and financial impact of disability, thereby improving the quality of life. This can be done through rehabilitation, which is the retraining of the remaining functions for maximal effectiveness.
Example: When a person becomes blind due to vitamin A deficiency, tertiary prevention (rehabilitation) can help the blind or partly blind person learn to do gainful work and be economically self supporting.
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