Chapter 14 Principles of Disease and Epidemiology

14. Principles of Disease

14.1 Definitions

  • Pathology: The study of disease.

  • Etiology: The cause of a disease.

  • Pathogenesis: The development of a disease.

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

  • Disease: An abnormal state in which the body is not performing normal functions.

14.2 Human Microbiome

  • Establishment: Begins in utero; influenced by food, people, and pets throughout life.

  • Human Microbiome Project: Analyzes relationships between microbial communities and health outcomes.

  • Types of Microbiota:

    • Normal microbiota: Permanently colonize the host and typically do not cause disease under normal conditions.

    • Transient microbiota: Present temporarily for days, weeks, or months.

14.3 Factors Affecting Microbiota Distribution

  • Determined by factors such as:

    • Nutrients, physical and chemical factors, host defense mechanisms, and mechanical factors.

14.4 Representative Normal Microbiota by Body Region

  • Skin: Microbes like Propionibacterium and Staphylococcus, which are limited by moisture content and antimicrobial secretions.

  • Eyes: Similar microbiota as skin; tears help eliminate some microbes.

  • Nose and Throat: Includes Staphylococcus and Streptococcus; nasal secretions prevent many pathogens.

  • Mouth: Hosts diverse microbiota due to abundant moisture and nutrients.

  • Large Intestine: Contains the largest numbers of resident microbiota due to moisture and nutrient availability.

  • Urinary and Reproductive Systems: Microbiota differ between urethra and vagina, environments inhibiting many pathogens.

14.5 Impact of Birth on Microbiome

  • Vaginal Birth: Results in microbiomes rich in Lactobacillus and Bacteroides.

  • Cesarean Birth: Microbiome resembles that of human skin, predominantly Staphylococcus aureus.

14.6 Microbial Antagonism and Symbiosis

  • Microbial Antagonism: Normal microbiota protect the host by competing for nutrients, producing harmful substances to invaders, and influencing pH/oxygen levels.

  • Symbiosis Types:

    • Commensalism: One benefits, the other is unaffected.

    • Mutualism: Both benefit.

    • Parasitism: One benefits at the other's expense.

  • Some normal microbiota may become opportunistic pathogens under certain conditions.

14.7 Koch’s Postulates

  • Core Principles:

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

    2. The pathogen must be isolated and grown in pure culture.

    3. The pure culture must cause the disease when introduced to a healthy lab animal.

    4. The pathogen must be isolated again from the inoculated animal.

  • Exceptions: Some pathogens cause multiple disease conditions, some are human-specific, and some cannot be cultured in labs.

14.8 Classifying Infectious Diseases

  • Symptoms: Felt changes via disease.

  • Signs: Measurable changes.

  • Syndrome: A specific group of signs and symptoms.

  • Types of Diseases:

    • Communicable: Spread between hosts.

    • Contagious: Easily and rapidly spread.

    • Noncommunicable: Not spread between hosts.

14.9 Disease Occurrence

  • Incidence: Number of new cases in a time period.

  • Prevalence: Total number of cases at a specific time.

  • Types:

    • Sporadic: Occasional occurrences.

    • Endemic: Constant presence in a population.

    • Epidemic: Rapid spread in a short time.

    • Pandemic: Global epidemic.

14.10 Stages of Disease Development

  • Incubation: Initial infection to first symptoms.

  • Prodromal: Early, mild symptoms.

  • Illness: Most severe stage.

  • Decline: Symptoms subside.

  • Convalescence: Recovery phase.

14.11 Reservoirs of Infection

  • Human Reservoirs: Carrier individuals.

  • Animal Reservoirs: Zoonoses (animal to human).

  • Nonliving Reservoirs: Soil and water sources.

14.12 Transmission of Disease

  • Contact Transmission: Direct, congenital, indirect, droplet.

  • Vehicle Transmission: Airborne, waterborne, foodborne.

  • Vector Transmission:

    • Mechanical: Pathogen on an organism's body.

    • Biological: Pathogen reproduces within the vector.

14.13 Healthcare-Associated Infections (HAIs)

  • HAIs: Acquired during healthcare treatment; affect 1 in 25 hospital patients leading to significant morbidity/mortality.

  • Prevention: Universal and transmission-based precautions are critical in controlling HAIs.