Principles of Disease and Epidemiology

Chapter 14: Principles of Disease and Epidemiology

Dr. Stuart

Learning Objectives

  • Define Epidemiology and the function of the CDC
  • List reasons for Emerging Infectious Diseases (EIDs)
  • List different reservoirs of infection and routes for disease transmission
  • List predisposing factors for diseases
  • Define terms for classifying occurrence of diseases
  • List Koch's Postulates and exceptions to the postulates
  • Define pathology, etiology, infection, and disease
  • Define the human microbiome (normal and transient microbiota)

Human Microbiome

Microbiome Overview
  1. What is the microbiome? It encompasses the trillions of microorganisms inhabiting the human body.
  2. Good or Bad Microbes? The microbiome is largely composed of beneficial microbes, playing crucial roles in health.
Microbial Colonization
  • Microbes begin colonization in and on the body after birth.
  • Sources of Acquisition: More microbes are acquired through food, people, and pets.
  • Colonization can be either temporary or permanent.
    • Normal microbiota: Permanently colonize the host and do not cause disease under normal conditions.
    • Transient microbiota: Present for days, weeks, or months.
Fun Facts
  • Approximately 100 trillion microbes reside on the human body.
  • Each person’s microbiome is unique, akin to a fingerprint.
  • Microbe genes outnumber human genomic genes by a ratio of 150:1.

Relationships Between Normal Microbiota and Host

Types of Symbiosis
  • Commensalism: One organism benefits while the other remains unaffected.
  • Mutualism: Both organisms benefit.
    • Example: Staphylococcus epidermidis bacteria on the skin.
  • Parasitism: One organism benefits at the expense of another.
    • Example: E. coli in the large intestine and influenza viruses on host cells.
Microbial Antagonism
  • Refers to competitive exclusion where normal microbiota protect the host by:
    1. Competing for nutrients with other microbes.
    2. Producing substances harmful to invading microbes.
    3. Altering pH and available oxygen conditions in the body.

Pathology, Infection & Disease

Definitions
  • Pathology: The study of disease.
    • Etiology: The cause of the disease.
    • Pathogenesis: The process of disease development.
  • Infection: Invasion and subsequent colonization (growth) of pathogens in a host.
  • Disease: An abnormal state where normal body functions are impaired due to pathogenic invasion.
    • Pathogens: Microorganisms that cause disease.
    • Interaction: Infection occurs when pathogens overcome the host’s defensive mechanisms.
Diagram Overview
  • Infection occurs in the host through the pathogen (virus, bacteria, fungi, parasite) and environmental factors (clinics, homes, travel, climate).

Etiology of Infectious Diseases

Koch’s Postulates
  • Established by Robert Koch, these are the criteria to prove a specific microbe causes a specific disease. They include:
    1. The same pathogen must be present in every case of the disease and absent from healthy hosts.
    2. The pathogen must be isolated and grown in pure culture from the diseased host.
    3. The pure culture pathogen must cause disease when introduced to a healthy, susceptible laboratory animal.
    4. The pathogen must be re-isolated from the inoculated animal and identified as the original organism.
Exceptions to Koch’s Postulates
  • Some pathogens can cause multiple disease conditions or may have asymptomatic carriers, making it difficult to apply these postulates directly.
  • Certain pathogens only cause diseases in humans and some haven’t been cultured yet.