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Understanding Disease Transmission and Epidemiology

Mar 18, 2025

Chapter 14: Disease and Epidemiology - Part 3

Overview

  • Focus: Study of epidemiology, specifically the movement of disease in a population from a microbe's perspective.
  • Process: Origin of microbe, transmission, entry into the body, causing disease, and exit.

Sources of Microbes (Reservoirs)

Nonliving Reservoirs

  • Soil: Habitat for fungi, Clostridium species (e.g., Clostridium tetani, Clostridium botulinum), Bacillus anthracis.
  • Water: Habitat for microbes like Entamoeba histolytica (causes travelers' diarrhea).

Living Reservoirs

  • Animals: Common reservoirs include pigs, poultry, bats.

    • Pigs and Poultry: Hosts for influenza viruses.
    • Bats: Reservoirs for rabies, Ebola, possibly COVID-19.

  • Humans: Most concerning in the US, especially children.

    • Children: Underdeveloped immune systems and poor hygiene.
    • Ill Humans and Carriers:
      • Ill individuals actively replicate and spread microbes.
      • Carriers like "Typhoid Mary" can transmit without being ill themselves.

Transmission of Microbes

Modes of Transmission

  1. Direct Contact: Physical contact including touching, kissing, bites.
  2. Indirect Contact: Transmission via fomites (inanimate objects like doorknobs, pens).
  3. Droplet Transmission: Through microscopic water droplets from exhalation, sneezing, or talking.
  4. Vehicle Transmission:
    • Water: Contaminated water spreads microbes (cholera, E. coli).
    • Food: Contaminated during butchering or handling (salmonella in poultry, E. coli in beef).
    • Air: Microbes travel distances >1 meter (e.g., smallpox).
    • Bodily Fluids: Blood, saliva, etc., significant in healthcare settings.
  5. Vector Transmission:
    • Mechanical: Microbes on insect bodies (flies).
    • Biological: Insect is infected and transmits via bites (mosquitoes: malaria, Zika; ticks: Lyme disease).

Entry and Exit of Microbes

Portals of Entry

  1. Mucous Membranes: Most common entry points.
    • Respiratory System: Inhalation of microbes.
    • Gastrointestinal System: Ingestion of food/water.
    • Urogenital System: Typically related to STDs.
  2. Skin: Contact with contaminated surfaces.
  3. Parenteral Route: Through injections beneath skin by insects.

Portals of Exit

  • Same as portals of entry, facilitating the spread to new hosts.

Epidemiology Review

  • Practice: Identify reservoir, transmission, and entry/exit points for common diseases using the microbes list.
  • Disease Classification:
    • Respiratory Diseases: Common cold, flu, COVID-19, measles.
      • Reservoir: Humans, Transmission: Contact/Droplet, Entry/Exit: Respiratory.
    • Gastrointestinal Diseases: Food poisoning (E. coli, Salmonella), cholera.
      • Reservoir: Water/animals, Transmission: Vehicle, Entry/Exit: Gastrointestinal.
    • STDs: Direct contact transmission.
    • Vector Borne: Insects as reservoirs, parenteral entry.

These notes encapsulate the lecture's main points, focusing on microbial movement through populations and the mechanisms of transmission and infection.

Full transcript