Causation and Theories of Disease

Causation (Role of Factors)

• Causation is defined as "anything producing an effect or a result."
• Medical textbooks discuss cause under headings like etiology, pathogenesis, mechanisms, and risk factors.
• Causal pathways can be direct or indirect.
  • Direct causation: A causes B without intermediate effects (rare).
  • Indirect causation: A causes B, but with intermediate effects.
• In human biology, intermediate steps are always present in any causal process.

Theories of Disease Causation

• Supernatural Theories: disease caused by curses, evil forces.
• Hippocratic Theory: disease from imbalances of four humors (yellow bile, black bile, phlegm, blood).
• Miasma Theory: disease caused by bad air or vapors.
• Contagion Theory: disease spread through contact or infection.
• Germ Theory: disease caused by microorganisms (via Henle-Koch postulates).
• Classic Epidemiological Theory: disease results from interactions of agent, host, and environment (Epidemiologic Triad).
• Multicausality and Webs of Causation: several factors influence disease occurrence (Bradford Hill's criteria).

Hippocratic Theory

• Hippocrates proposed disease results from an imbalance among four vital humors (yellow bile, black bile, phlegm, and blood).
• Imbalance in any humor can lead to health deterioration and development of symptoms.
• Hippocrates observed and connected individual exposures (diet, exercise, occupation) with health outcomes.

Henle-Koch Postulates (Germ Theory)

• While miasmatic theory focused on environmental causes, the germ theory gained acceptance.
• Louis Pasteur introduced the germ theory (later refined as Henle-Koch postulates).
• Postulates:
  • The infectious agent must be found in every case of the disease.
  • It must not be found in cases of other diseases.
  • The agent must be isolated and grown in pure culture.
  • Introduction of the agent into healthy, susceptible subjects must cause the disease.

Classic Epidemiologic Theory: Epidemiologic Triad

• Infectious diseases are caused by the interaction between agent, host, and the environment.
• Agent: the causative factor (microorganism, chemical, or physical).
• Host: the susceptible individual (with characteristics like genetic makeup, immunity).
• Environment: external factors influencing exposure (climate, sanitation, and social factors).

Classic Epidemiologic Theory (Continued)

• Agent refers to infectious microorganisms, and can be broadened to include chemicals, physical forces, and nutritional deficiencies.
• Infectivity: proportion of exposed people who become infected.
• Pathogenicity: proportion of infected developing clinically apparent disease.
• Virulence: proportion of clinically apparent cases severely or fatally affected.

Classic Epidemiologic Theory (Continued)

• Host refers to the human who can get the disease.
• Factors intrinsic to the host (risk factors) can influence exposure, susceptibility, or response to the agent.
• Exposure opportunities are influenced by behaviors (sexual, hygiene, smoking), personal choices, and age/sex.
• Susceptibility and response to agents are affected by genetic composition, nutritional/immunologic status, anatomic structure, presence of other diseases, and psychological makeup.

Classic Epidemiologic Theory (Continued)

• Environment refers to extrinsic factors influencing exposure and the factors that affect the agent.
• This includes physical conditions (geology and climate), biologic conditions (insects transferring the agent), and socioeconomic factors (crowding, sanitation, availability of healthcare).

Factors Associated with Increased Risk of Human Disease

• Tables provide a breakdown of host, agent, and environmental factors associated with increased risk of human disease.

Causation (Continued)

• The concept of necessary and sufficient factors is explained (with examples).
• Necessary and Sufficient
  • Factor A alone causes disease
• Necessary but not Sufficient
  • Multiple factors needed in a specific order to cause disease.
• Sufficient, but not Necessary
  • Multiple factor that can cause the disease.
• Neither Sufficient nor Necessary
  • Many factors cause the disease.

Multicausal Theories

• While the epidemiologic triad is useful, it's insufficient for many complex diseases (like cardiovascular disease and cancer).
• Other models that account for multifactorial disease causation have been proposed.

Concept of Disease Occurrence-Etiology

• Etiology is the sum of all factors (agent, host, and environment) that contribute to disease occurrence.
• Modifiable factors are most important in disease prevention.

This quiz explores the fundamental concepts of causation in human biology and various theories that explain disease causation. From supernatural beliefs to modern germ theory, test your knowledge of how diseases are understood within different frameworks. Ideal for students studying medical sciences and epidemiology.