Concept of Disease in Public Health

Concept of Disease

• A disease is a condition where body function is impaired, departing from a state of health, and altering the performance of vital functions.

Theories of Disease Causation

• Supernatural theory of disease
• Ecological theory
• Germ theory: Microorganisms cause many known diseases (Pasteur, Henle, Koch)
• Multifactorial causation theory: Agent, host, and environmental factors interact synergistically to cause disease

Epidemiologic Triad

• Agent (pathogen): Biological, Nutrient, Chemical, Physical, Mechanical, or Genetic
• Host (susceptible individual): Intrinsic (age, race, sex, socioeconomic status) and modifiable factors (behaviors)
• Environment: Physical (geology, climate), Biologic (insects), and Socioeconomic (crowding, sanitation, health services)

Natural History of Disease

• Refers to the progress of a disease process in an individual over time without 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 a population
• Varying degrees of severity due to interactions of host, agent, and environmental factors
• Disease forms:
  • Acute
  • Chronic/insidious
  • Carrier

Iceberg Phenomenon

• Cases of illness diagnosed by clinicians represent only the "tip of the iceberg"
• Many additional cases may be too early to diagnose or remain asymptomatic
• Risk of transmission by persons with in-apparent or undiagnosed infections