Chapter 14: Principles of Disease and Epidemiology

Chapter 14: Principles of Disease and Epidemiology

Key Vocabulary and Concepts

  • Epidemiology: Study of the incidence, distribution, and control of disease in a population.

    • Key Questions: Who, Where, How, What, When, Why (W5H)

  • Pathogen: A disease-causing organism or agent.

  • Pathology: Study of structural and functional changes associated with disease.

  • Pathogenesis: The development process of a disease and the means by which it is caused.

  • Infection:

    • Definition: The invasion of a host and evasion of defenses.

    • State of Condition: Body invaded by a pathogenic organism or agent.

  • Disease: Departure from normal structure or function.

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Etiology & Koch's Postulates

  • Etiology: Study of the causative agent of disease.

  • Koch's Postulates:

    1. Microorganisms are isolated from a diseased or dead animal.

    2. Microorganisms are grown in pure culture.

    3. Microorganisms are inoculated into a healthy laboratory animal, reproducing the disease.

    4. Microorganisms are isolated from this animal and identified.

Symbiotic Relationships

  • Types of Symbiosis:

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

    • Mutualism: Both organisms benefit.

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

Normal Microbiota and the Host

  • Normal Flora/Microbiota Locations:

    • Nose and throat (upper respiratory)

    • Eyes (conjunctiva)

    • Mouth

    • Skin

    • Large intestine

    • Urinary and genital systems (lower urethra and vagina)

  • Opportunistic Pathogens: Normally do not cause disease but can under certain conditions.

Signs and Symptoms of Diseases

  • Signs: Quantitative/measurable effects (e.g., fever, rash).

  • Symptoms: Non-quantitative/subjective effects (e.g., pain, nausea).

  • Syndrome: A specific set of signs and symptoms that occur together in a disease condition.

Types of Diseases

  • Communicable Disease: Can be transmitted from host to host.

  • Noncommunicable Disease: Cannot be spread under ordinary conditions.

Epidemiological Terms

  • Incidence: Number of new cases over a given time in a defined population.

  • Prevalence: Total number of cases (new and pre-existing) at a given time.

  • Disease Classification:

    • Endemic: Always present in an area, usually low numbers.

    • Epidemic: High number of cases in a short time in a particular area.

    • Pandemic: Global epidemic.

Severity and Duration of Diseases

  • Acute: Rapid onset and short duration.

  • Chronic: Develops slowly and lasts a long time.

  • Subacute: Intermediate between acute and chronic.

  • Latent: Signs/symptoms disappear but pathogen is still present.

Extent of Involvement

  • Local: Effects confined to a small area.

  • Systemic: Infection spread throughout the body.

  • Focal: Local infection spreads to a different area.

  • Primary Infection: Initial infection causing illness.

  • Secondary Infection: Opportunistic infection following a primary one.

Terms Associated with Infection Spread

  • Reservoir: Continual source of a pathogen.

  • Carrier: Infected individual with no or mild symptoms who spreads the disease.

  • Zoonotic Disease: Disease passed from animals to humans.

Selected Zoonoses

  • Bacterial:

    • Anthrax: Bacillus anthracis via contaminated hides/animals.

    • Brucellosis: Brucella spp. from contaminated milk or meat.

  • Viral:

    • Influenza: Influenzavirus from swine/birds.

    • Rabies: Lyssavirus from bites of infected animals.

  • Protozoal:

    • Malaria: Plasmodium spp. via Anopheles mosquito bite.

Infection Transmission Modes

  1. Contact Transmission:

    • Direct: Person-to-person.

    • Indirect: Touching contaminated objects.

    • Droplet: Inhalation or contact with droplets.

  2. Vehicle Transmission:

    • Waterborne.

    • Foodborne.

    • Airborne.

  3. Vector Transmission:

    • Mechanical (e.g., on insect feet).

    • Biological (e.g., bites).

Development of Disease Stages

  1. Incubation: Time between infection and appearance of symptoms.

  2. Prodromal Period: Mild signs and symptoms.

  3. Period of Illness: Full-blown signs and symptoms.

  4. Period of Decline: Decreasing severity of symptoms.

  5. Period of Convalescence: Regaining strength and health.

Fields of Epidemiology

  • Descriptive: Collection and analysis of disease occurrence data.

  • Analytical: Determining probable causes via comparisons and associations.

  • Experimental: Controlled laboratory experiments to understand disease causes.