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    • Chapter 7 Pearson Homework due Monday, March 2 by 11:59 pm
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    • Dynamic Study Module for Chapter 9 has been posted

Textbook Information

  • Second Edition
    • Title: Microbiology: Basic and Clinical Principles
    • Author: Lourdes Norman-McKay

Chapter 9 – Principles of Infectious Disease and Epidemiology

Causes of Infectious Diseases

Learning Objectives

  • List the six different categories of pathogens.
  • Compare the following sets of terms:
    • Opportunistic versus true pathogen
    • Endemic vs. sporadic disease
    • Communicable vs. noncommunicable disease
    • Acute vs. chronic disease
  • Explain the differences between an epidemic and a pandemic.
  • State how an emerging disease differs from a reemerging disease.
  • Explain Koch’s postulates of disease and how they are used.

Disease Terminology

Definitions

  • Infectious Disease: An illness caused by a pathogen.
  • Epidemiology: The monitoring and controlling of disease occurrence to promote public health.
  • Pathogens include:
    • Prions
    • Viruses
    • Bacteria
    • Protozoans
    • Helminths
    • Fungi

Pathogen Definitions

  • Opportunistic Pathogens: Only cause disease when the host is weakened;
    • Example: Candida albicans
  • True Pathogens: Do not require a weakened host to cause disease.

Disease Classification

Types of Occurrences

  • Sporadic Cases: Isolated infections in a particular population (e.g., Ebola).
  • Endemic Infections: Routinely detected in a population or region (e.g., common cold viruses).
  • Epidemic: A widespread disease outbreak in a particular region during a specific time frame.
  • Pandemic: If an epidemic spreads to numerous countries/continents (e.g., COVID-19).

Emerging and Reemerging Pathogens

Classifications

  • Emerging Pathogens: Newly identified agents and pathogens previously causing only sporadic cases or exhibiting expanded geographical distribution (e.g., SARS-CoV-2, Zika virus).
  • Reemerging Pathogens: Infectious agents that were under control but are resurfacing (e.g., antibiotic-resistant bacteria like Mycobacterium tuberculosis).

Zoonotic Diseases

  • Since the 1970s, approximately 60% of emerging infections characterized are zoonotic diseases, which spread from animals to humans.
  • Many are noncommunicable (do not spread from person to person), while communicable diseases transmit from human to human, and contagious diseases are easily transmitted.

Infection and Symptoms

Definitions

  • Active Infection: The patient is symptomatic, exhibiting signs and symptoms.
    • Signs: Objective indicators of disease (e.g., fever, rash, blood in stool).
    • Symptoms: Subjective experiences of the patient (e.g., pain, fatigue, nausea).
  • Latent Infection: Usually asymptomatic (no signs or symptoms).

Onset and Duration

  • Acute Diseases: Rapid onset and progression.
  • Chronic Diseases: Slower onset and progression.

Koch’s Postulates of Disease

Overview

  • Koch’s postulates allowed for the identification of causative pathogens of various infectious diseases.

Limitations

  • Do not apply to noninfectious diseases.
  • Certain infectious diseases cannot be isolated.
  • Infectious agents that won’t grow in the lab.
  • Obligate intracellular pathogens.
  • Some microbes may become attenuated in pure cultures.
  • Not easily applied to infectious agents primarily causing latent disease.

Infectious Disease Transmission and Stages

Learning Objectives

  • Define the terms reservoir, source, and transmission, and contrast endogenous and exogenous sources.
  • Describe various direct and indirect modes of transmission, providing examples of each.
  • Compare and contrast biological and mechanical disease vectors.
  • Explain the five stages of infectious disease, summarizing how these vary by pathogen and the challenges presented in epidemiology.
  • Compare chronic carriers and asymptomatic carriers.

Sources of Pathogens

Definitions

  • Reservoirs: An animate or inanimate habitat where the pathogen is naturally found.
  • Sources: Disseminate (spread/distribute) infectious agents from the reservoir to new hosts.
    • Endogenous Source: Pathogen comes from the host’s own body.
    • Exogenous Source: Pathogen is external to the host.
    • Example: Clostridium botulinum
      • Reservoir: Soil
      • Source: Contaminated food

Examples of Sources

  • Exogenous Sources:
    • Environmental: Contaminated food, medical equipment, soil, water.
    • Animals: Transmit zoonotic diseases to humans.
    • Humans: Transmit communicable infections from one another.
    • Misplaced normal microbiota can lead to infections after entering surgical incisions.
    • Disrupted microbiota can lead to opportunistic infections (e.g., yeast infections following antibiotic treatment).

Transmission of Pathogens

Modes of Transmission

  • Direct Contact Transmission: Pathogen spread through physical contact:
    • Person to person (e.g., touching, kissing, bodily fluids).
    • Animal interactions (e.g., bites or scratches from vectors).
  • Indirect Contact Transmission: No direct contact; includes:
    • Airborne transmission (e.g., respiratory droplets).
    • Vehicle transmission (e.g., contaminated objects, food, or water).
    • Examples: COVID-19 via respiratory droplets, Salmonellosis through contaminated food.
    • Environmental sources.
    • Biological and mechanical vectors:
    • Biological: Part of the pathogen’s life cycle in the vector (e.g., malaria via mosquito).
    • Mechanical: Spreads disease without being integral to pathogen life cycle (e.g., flies transferring germs via contact with feces to food).
    • Vertical Transmission: From mother to child through various means during pregnancy and delivery.

Five General Stages of Disease

Description of Stages

  1. Incubation Period: Time between infection and symptom development.
  2. Prodromal Phase: Initial mild symptoms.
  3. Acute Phase: Patient experiences full-blown symptoms (asymptomatic cases may also exist).
  4. Period of Decline: Pathogen replication decreases, patient starts feeling better.
  5. Convalescent Period: Elimination of the pathogen; host may harbor the pathogen indefinitely or remain asymptomatic for extended periods.
  • Asymptomatic carriers can transmit pathogens, e.g., “Typhoid Mary” (Salmonella enterica serovar Typhi).

Epidemiology Essentials

Learning Objectives

  • Define epidemiology and state its two primary goals.
  • Describe host and environmental factors forming an epidemiological triangle, and provide examples of how it can be broken.
  • Explain the concept of quarantine and when it may be effective in limiting disease.
  • Describe three roles of public health.

Overview of Epidemiology

  • Epidemiology: “Study of what is upon the people,” focusing on diseases (both infectious and noninfectious) within populations.
  • Goals of Epidemiology:
    1. Describe the nature, cause, and extent of new or existing diseases in populations.
    2. Intervene to protect and improve health in populations.

The Epidemiological Triangle

Components

  • Components:
    • Host
    • Etiological Agent
    • Environment
  • The epidemiological triangle emphasizes the relationship between the host and the environment, sometimes being more significant than the specific causative agent.
  • Many pathogens have a limited host range and can evolve over time.
  • Host factors impacting disease progression include health status, age, and nutrition.

Public Health and Disease Prevention

Roles

  • Public Education: Essential for breaking the epidemiological triangle; includes vaccination campaigns, prenatal care promotion, and education about sexually transmitted diseases.
  • Quarantine: Useful for controlling diseases with short incubation periods but may be impractical for diseases with long incubation periods.
  • Vector Control: Effective in limiting numbers of biological vectors (e.g., fleas, mosquitoes, ticks) to prevent the spread of infectious diseases.

Agencies and Regulation

  • Public health responsibilities span local, regional, and national levels, with the Centers for Disease Control and Prevention (CDC) as a pivotal federal agency for epidemiological information.