Principles of Disease and Epidemiology Notes

Principles of Disease and Epidemiology
Learning Objectives

  • Cite four reasons why an individual might not develop an infectious disease after exposure to a pathogen.

  • Differentiate between localized and systemic infections.

  • Discuss the four periods or phases in the course of an infectious disease.

  • Explain how acute diseases differ from subacute and chronic diseases.

  • Briefly describe the three lines of defense used by the body to combat pathogens and give one example of each.

Key Terminology

  • Pathology: The study of a disease.

  • Etiology: The study of the cause of a disease.

  • Pathogenesis: The development of a disease.

  • Infection: Invasion and colonization of the body by pathogens.

  • Disease: An abnormal state in which the body is not functioning normally.

Normal Microbiota and the Host

  • Transient Microbiota: Present for days, weeks, or months.

  • Normal Microbiota: Permanently colonize the host.

  • Symbiosis: The relationship between normal microbiota and the host, including:

    • Commensalism: One organism benefits, the other is unaffected.

    • Mutualism: Both organisms benefit.

    • Parasitism: One organism benefits at the expense of the other.

  • Some normal microbiota can become opportunistic pathogens.

Microbiota Composition

Microbiota Regions:

  • Skin: Acts like a barrier with diverse microorganisms.

  • Oral Cavity: Hosts different microbiota important for health.

  • Gut: Critical microbiota that aid digestion and immunity.

  • Vagina: Predominantly Lactobacilli for maintaining pH.

Protective Functions of Normal Microbiota

  • Occupy niches that pathogens might otherwise occupy.

  • Produce acids and compete for nutrients.

  • Produce bacteriocins or antibiotics against invading microbes.

  • Probiotics: Live microbes applied to or ingested into the body intended to exert a beneficial effect.

Koch’s Postulates (Criteria for Identifying Pathogens)

  1. The same pathogen must be present in every case of the disease.

  2. The pathogen must be isolated from the diseased host and grown in pure culture.

  3. The pathogen from the pure culture must cause the disease when inoculated into a healthy, susceptible laboratory animal.

  4. The pathogen must be isolated from the inoculated animal and must be shown to be the original organism.

Classifying Infectious Diseases

  • Communicable Disease: Spread from one host to another.

  • Contagious Disease: Easily spread from one host to another.

  • Noncommunicable Disease: Not transmitted from one host to another.

Disease Occurrence and Epidemiology

  • Incidence: Fraction of a population that contracts a disease during a specific time.

  • Prevalence: Fraction of a population having a specific disease at a given time.

  • Sporadic Disease: Occurs occasionally in a population.

  • Endemic Disease: Constantly present in a population.

  • Epidemic Disease: Affected many hosts in a specific area in a short time.

  • Pandemic Disease: A worldwide epidemic.

  • Herd Immunity: Immunity in most of a population.

Factors Involved in Disease Interaction

  1. Pathogen Factors:

    • Virulence of the pathogen

    • Portal of entry to the body

    • Number of organisms entering the body

  2. Host Factors:

    • Health and nutritional status

    • Individual susceptibility

  3. Environmental Factors:

    • Sanitary conditions, availability of resources, etc.

Events in Infection

  1. Entry through correct portal of entry (e.g., mucous membranes, skin).

  2. Sufficient number of cells to evade defenses (Infectious Dose - ID).

  3. Predisposing factors: gender, weather, nutrition, etc.

Disease Progression Phases

  1. Incubation Period: Time after pathogen entry, before symptoms appear.

  2. Prodromal Period: Pathogen multiplies; early signs and symptoms appear.

  3. Period of Illness: Signs and symptoms specific and severe.

  4. Period of Decline: Immune response activated; pathogens decrease.

  5. Period of Convalescence: Recovery and return to normal function.

Infections Types

  • Local Infection: Limited to a small area.

  • Systemic Infection: Spread throughout the body.

  • Focal Infection: Initial local infection that spreads systemically.

  • Sepsis: Toxic inflammatory condition from spread of microbes.

Severity of Disease

  • Acute: Rapid onset but short duration.

  • Chronic: Develops slowly; lasts for months or years.

  • Subacute: Disease between acute and chronic.

  • Latent: Inactive stage with potential for reactivation.

Host Defense Mechanisms

  • First Line of Defense: Skin, mucous membranes, secretions (barrier immunity).

  • Second Line of Defense: Phagocytic leukocytes, inflammatory response, fever (innate immunity).

  • Third Line of Defense: Lymphocytes and antibodies (adaptive immunity).

Types of Immunity

  • Humoral Immunity: Involves antibody production by B-cells.

  • Cell-Mediated Immunity: Involves T-cells; targets infected or abnormal cells.

Antibodies

  • IgG, IgA, IgM, IgD, IgE: Different classes with specific functions in immune response.

Key Epdemiology Figures

  • John Snow: Mapped cholera incidence in London.

  • Ignaz Semmelweis: Demonstrated handwashing to reduce fever incidence.

  • Florence Nightingale: Showed improved sanitation decreases epidemic typhus incidence.

Conclusion

Epidemiology is crucial for understanding disease transmission and prevention strategies.