Epidemiology Chapter 1: The Epidemiologic Triad

The Epidemiologic Triad

• The epidemiologic triad consists of three components: an external agent, a susceptible host, and an environment that brings the host and agent together.
• The environment influences the agent, the host, and the route of transmission of the agent from a source to the host.

Disease Agents

• Biological Agents:
  • Viruses
  • Bacteria
  • Fungi
  • Protozoa
  • Helminthes
  • Arthropods
• Nutrient Agents:
  • Carbohydrates
  • Fats
  • Proteins
  • Minerals
  • Vitamins
  • Water
• Chemical Agents:
  • Endogenous chemical agents (formed in the body itself)
  • Exogenous Chemical Agents (enter the body from outside)
• Physical Agents:
  • Heat
  • Cold
  • Humidity
  • Pressure
  • Radiation
  • Electricity
  • Sound (noise)
• Mechanical Agents:
  • Chronic friction or other mechanical forces which result in injury, sprain, dislocation, or fracture
• Genetic Disorders:
  • Caused by genetic disorders and lack or under-development of organs

Host Factors

• Intrinsic Factors (non-modifiable):
  • Age
  • Race
  • Sex
  • Socioeconomic status
• Behaviors (modifiable factors):
  • Smoking
  • Drug abuse
  • Lifestyle
  • Sexual practices
  • Contraception
  • Eating habits

Environmental Factors

• Physical factors:
  • Geology
  • Climate
• Biologic factors:
  • Insects that transmit the agent
• Socioeconomic factors:
  • Crowding
  • Sanitation
  • The availability of health services

Natural History of Disease

• Refers to the progress of a disease process in an individual over time, in the absence of intervention.
• Stages of disease:
  • Pre-pathogenic phase or susceptibility stage
  • Pathogenic phase
    • Incubation period
    • Prodromal stage
    • Stage of overt disease
    • Stage of convalescence

Spectrum of Disease

• The idea that an exposure can lead to varying signs, symptoms, and severity of the same disease in the population.
• The outcome will depend on the interactions of host, agent, and environmental factors.
• Disease in many forms:
  • Acute
  • Chronic/insidious
  • Carrier

Iceberg Phenomenon

• Cases of illness correctly diagnosed by clinicians in the community often represent only the “tip of the iceberg.”
• Many additional cases may be too early to diagnose or may remain asymptomatic.
• Examples: Tuberculosis, meningitis, polio, hepatitis A, AIDS.
• Risk: Persons with in-apparent or undiagnosed infections may be able to transmit infection to others.

Applications of the Concepts of Natural History and Spectrum of Disease

• Control measures must not be directed solely for clinically apparent cases.
• Control measures must be directed toward all infections capable of being transmitted to others; both clinically apparent cases and those with in-apparent or undiagnosed infections.

Learn about the traditional model of infectious disease causation, including the three components of the epidemiologic triad: agent, host, and environment. Understand how the environment influences the agent, host, and route of transmission.