Epidemiology Principles: Disease Transmission and Causation

Posted on May 12, 2026 in Microbiology

Aim and Purpose of Epidemiology

  • Describe the distribution and magnitude of health and disease problems in human populations.
  • Identify etiological factors (risk factors) in the pathogenesis of disease.
  • Provide data essential for the planning, implementation, and evaluation of services for the prevention, control, and treatment of disease, and for setting priorities among those services.

Scope of Epidemiology

  1. Describe the spectrum of disease
  2. Identify the natural history of disease
  3. Community diagnosis
  4. Planning and evaluation
  5. Syndrome identification
  6. Searching for causes and risk factors
  7. Investigate epidemics of unknown etiology
  8. Explain the mechanism of disease transmission
  9. Screening
  10. Detection and control of infectious diseases
  11. Measurement of health status
  12. Research

Epidemiological Measurement

Tools of Measurement

  • Count, Ratio, Proportion, Rate

Scope of Measurements

  • Measurement of Mortality: Crude Death Rate (CDR), Infant Mortality Rate (IMR), Maternal Mortality Rate (MMR), Case Fatality Rate (CFR).
  • Measurement of Morbidity: Incidence and Prevalence.
  • Measurement of Disability: DALY (Disability Adjusted Life Years) for total disease burden.
  • Measurement of Natality (Birth): Crude Birth Rate (CBR) and Fertility Rate.
  • Measurement of Disease Characteristics
  • Measurement of Health Services
  • Measurement of Environment and Risk Factors
  • Measurement of Demographic Variables

Descriptive Epidemiology

Characteristics

  • Time Distribution: Short-term, periodic, and long-term (secular) trends.
  • Place Distribution: International, national, and rural–urban variations.
  • Person Distribution

Procedures and Steps

  1. Defining the population
  2. Defining the disease under study
  3. Describing disease by time, place, and person
  4. Measurement of disease
  5. Comparison with known indices
  6. Formulation of etiological hypothesis

Uses

  • Determining disease magnitude
  • Defining distribution of disease
  • Formulation of etiological hypothesis
  • Identification of high-risk groups
  • Planning and evaluation of health services
  • Basis for further research

Chain of Infection and Disease Transmission

Components of the Chain

  1. Source and Reservoir of Infection: The source is the person, animal, or object from which an infectious agent passes to the host. The reservoir is the habitat (person, animal, plant, soil) where the agent lives and multiplies.
  2. Mode of Transmission
  3. Susceptible Host

Types of Reservoirs

  • Human, animal, and non-living reservoirs.

Case Definitions

  • By Chronology: Primary, index, and secondary cases.
  • By Clinical Manifestation: Clinical, sub-clinical, and latent.

Carrier Classifications

  • By Stage: Incubatory, convalescent, and healthy.
  • By Duration: Temporary and chronic.
  • By Portal of Exit: Intestinal, urinary, respiratory, nasal, and blood/wound.

Modes of Transmission

Direct Transmission

  • Direct contact, droplet infection, contact with infected soil, inoculation into skin/mucosa, and vertical transmission.

Indirect Transmission

  • Vehicle-borne (food, water, blood), vector-borne (insects), air-borne (droplet nuclei/dust), fomite-borne, and unclean hands.

Concept of Disease Causation

Ancient Theories

  • Supernatural, theory of humors, and miasmatic theory.

Modern Concepts

  • Germ Theory: Specific microorganisms cause specific diseases.
  • Multifactorial Causation: Disease results from agent, host, and environment interactions.
  • Web of Causation: Complex interaction of many factors.
  • Epidemiological Triad: Interaction between Agent, Host, and Environment.

The Epidemiological Triad

Components

  • Agent Factors: Biological, physical, mechanical, social, chemical, and nutritional.
  • Host Factors: Demographic, lifestyle, social, economic, and biological.
  • Environmental Factors: Physical, psycho-social, and biological.