epidemiology
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THE EPIDEMIOLOGICAL TRIAD
The occurrence and manifestations of any disease whether communicable or non communicable are determined by the interaction between the
  1. The Agent
  2. The Host &
  3. The Environment
Which together constitute the epidemiological triad.
THE AGENT
The Agent is defined as “an organism, a substance or a force, the presence or lack of which may initiate a disease process or may cause it to continue. They may be classified as
  • Living
  • Non-Living
Living or Biological Agents e.g.,
  • Bacteria
  • virus
  • Fungi
Non-Living or Inanimate
  1. Nutrient Agents
  2. Chemical Agents &
  3. Physical Agents
Nutrient Agents :-- e.g.,
  • Protein
  • Fats
  • Carbohydrates
Chemical Agents
  1. External e.g., Lead, Arsenic
  2. Internal e.g., Urea in renal failure, ketone bodies in diabetes
Physical Agents e.g.,
  • Atmospheric Pressure, Temperature
HOST
The host is the man himself. The characteristics of a human being that determine how he reacts to the agents in the environment are called “host factors”. The host factors are
  • Demographic Characteristics
  • Biological Characteristics
  • Socio economic Characteristics
  • Life style
1. Demographic Chacteristics e.g.
  • Age,
    Sex,
    Ethnicity ( race )
2. Biological Chacteristics
  • Genetic Background
  • Immune status
  • Nutritional Status
3. Socio Economic Characteristics
  • Social class
  • Religion
  • Education
  • Martial Staus
4. Life Style
  • Living habits
  • Food habits
In terms of infectious disease epidemiology , Host is defined as “a person or an animal that afford subsistence or lodgment to an infectious agent under natural conditions.
THE ENVIRONMENT
The environment of man is of two types
  • Internal &
  • External
Internal environment is comprised by the various organs and organ systems within the human bodies.
External environment is defined as” all that which is external to the individual human host “.
Environment is the source or reservoir for the agent of the disease. It helps in the transmission of the agent to the host , bringing about their contact and interaction Generally the environment is studied under three headings
  1. Physical environment
  2. Biological environment
  3. Social environment
Physical environment is the space around man containing gases. Liquids and solids.
Biological environment means the universe of all living things that surround man. It comprises both animals and plants
1. They may be reservoirs of disease germs ( rats in case of plague )
2. They may be transmitters of disease agents ( mosquitoes )
3. They may be causative agents of disease ( bacteria or virus ).
Social environment comprises all human beings around man and their activities and interactions. It may be considered under social and economic factors
a. social factors pertain to the society in which man lives. They provide stimuli that effect the physical, mental and social state of man to which he must adjust, e.g.,
  • The customs of society
  • Attitude of the office boss
b. economic factors refer to the material assets and gains of the human society. They determine the economic status of the man which in turn affects his health. Thus low economic status means less diet, poor housing and less resources for medical aid.