Introduction to Infectious Diseases: Terms and Concepts

Course Overview and Administrative Information

  • Institution: Ross University School of Veterinary Medicine (RUSVM).
  • Course: Principles of Infectious Diseases (PID).
  • Course Context: Infectious diseases have historically killed more humans than all wars combined throughout the entire history of humankind.
  • The PID Instructional Team:     * Dr. Ricardo Gutiérrez (Lic. Microb., MSc., Ph.D.): Assistant Professor of Veterinary Bacteriology and Course Coordinator.     * Dr. George Nadas: Immunology (locum).     * Dr. Pithua: Epidemiology.     * Dr. Ghosh: Virology.     * Dr. Yao: Parasitology.
  • Course Structure and Grading Weights:     * Block 1: Baseline weight.     * Block 2: Baseline weight.     * Block 3: +10−25%+ 10-25\% above previous block weight.     * Block 4: +10−25%+ 10-25\% above previous block weight.     * Block 5: +10−25%+ 10-25\% above previous block weight.     * Block 6: +10−25%+ 10-25\% above previous block weight.
  • Learning Objectives:     * Objectives are stated per lecture or topic and serve as guidelines.     * Specifics are presented by each professor during individual lectures.
  • Helpful Student Tips:     * Keep up with the information continuously.     * Ask questions frequently.     * Collaborate with peers and Teaching Assistants (TA).     * Engage with different teaching styles from various professors for enrichment.     * Reach out via email or schedule one-on-one/group meetings if needed.
  • Primary Literature/Resources:     * Main Textbook: Veterinary Microbiology and Microbial Disease, Second Edition (PJ Quinn, B K Markey, F C Leonard, E S FitzPatrick, S Fanning, P J Hartigan).     * Course material including lecture slides and background information is hosted on CANVAS.

History and Foundations of Microbiology

  • Microorganism or Microbe Definition: Any organism that is invisible to the naked eye.
  • First Observation: Antonie van Leeuwenhoek first observed microorganisms in the 1670s.
  • Key Historical Figures and Contributions:     * Agostino Bassi (1835): Demonstrated that "muscardine," a disease in silkworms, was contagious and caused by the microscopic fungus Beauveria bassiana.     * Louis Pasteur (1850s–1880s):         * Studied the role of microbes in fermentation and disease pathology.         * Developed the pasteurization process.         * Developed vaccines against rabies and anthrax.     * Robert Koch (1860s–1880s):         * Discovered the specific bacteria responsible for anthrax, tuberculosis, and cholera.         * Developed the fundamental "Koch’s Postulates."

Classification and Scale of Infectious Agents

  • Biological Scale (Smallest to Largest):     1. Prions (Smallest)     2. Viruses     3. Bacteria     4. Fungi     5. Protozoa     6. Multicellular parasites (Largest, reaching up to 1 cm1\,cm and beyond).
  • Ecological Classification: Microorganisms and parasites can be classified based on their ecological niche—specifically their ability to multiply inside and outside hosts (animals, humans, arthropods):     * Opportunistic/Facultative: Organisms that can live independently but may cause disease under specific conditions.     * Obligate: Organisms that require a host to complete their life cycle or multiply.
  • Microbiome: A microbiota built by a diverse collection of fungi, bacteria, archaea, viruses, and mites that cover animal tissues (e.g., skin, gastrointestinal tract, respiratory tract).

Core Terms and Concepts in Infectious Disease

  • Infectious Disease: Disorders caused by organisms such as bacteria, viruses, fungi, or parasites.
  • Communicable Disease: Any infectious disease that spreads from individual to individual. This may require a specific mode of transmission (e.g., vector transmission or sexual transmission) through direct or indirect routes.
  • Contagious Disease: Infectious diseases that spread easily from one individual to another through air or physical touch.
  • Conceptual Distinctions:     * Infectious does not always mean transmissible (Example: Tetanus).     * Communicable refers to a transmissible infectious disease (direct or indirect).     * Contagious refers to easily transmissible diseases (direct contact or close proximity).     * Hierarchical Logic: All contagious diseases are communicable; all communicable and contagious diseases are infectious. However, not all infectious diseases are transmissible (communicable), and not all communicable diseases are contagious.
  • Notifiable (Reportable) Disease:     * Legally mandated to be reported to public health authorities to control or prevent spread.     * "Notifiable" usually refers to national jurisdiction.     * "Reportable" varies across local, state, and national levels.     * Note: Not all notifiable diseases are infectious (Example: Cancer, lead poisoning, congenital conditions).
  • Zoonotic Disease:     * A pathogen/microorganism that is a commensal or pathogen in animals but can be transmitted to humans to cause disease.     * Transmission occurs through vectors (e.g., ticks), direct contact with the animal, or contact with its bioproducts.

The Infectious Disease Triad

Infectious disease is the result of interactions between three primary components:

  • Pathogen Factors:     * Pathogenicity.     * Dispersal efficiency.     * Survival efficiency.     * Inoculation dose.
  • Environment Factors:     * Abiotic and biotic factors.     * Housing management.     * Population density.     * Sanitation.     * Nutrition.     * Prevention strategies.
  • Susceptible Host Factors:     * Breed, age, sex, and genotype.     * Physiology and individual susceptibility.     * Immune response status.

Exposure Outcomes and Disease Progression

  • The Exposure Continuum: Exposure → Infection → Disease.
  • Potential Outcomes of Exposure:     * No Infection: Pathogen fails to colonize.     * Infection with Sub-clinical outcome: Results in either pathogen elimination or the host becoming a Carrier (immunity or non-immunity).     * Infection with Clinical outcome:         1. Recovery (Pathogen elimination and immunity).         2. Carrier state (Persistence of pathogen).         3. Death or Disability.
  • Types of Carriers:     * Active Carriers: Currently shedding or harboring the pathogen.     * Latent Carriers: Pathogen is present but inactive; may become active later.
  • Stages of Disease Progression:     1. Exposure: Contact with the agent.     2. Incubation Period: Time between exposure and first signs.     3. Prodromal Period: Early, mild, non-specific signs/symptoms.     4. Illness: Significant signs and symptoms of disease.     5. Outcome: Either death, recovery, or transition to a latent/persistent carrier state at the end of symptoms.

Koch’s Postulates

  • Purpose: To determine if a specific disease is infectious and to identify the etiological (causative) agent.
  • The Four Postulates (1884):     1. The suspected pathogen must be found in every case of the disease and must not be found in healthy individuals.     2. The suspected pathogen must be isolated from the diseased host and grown in a pure culture.     3. The same disease (signs and symptoms) must develop when a healthy, susceptible individual is inoculated with the cultured pathogen.     4. The pathogen must be re-isolated from the newly diseased host and identified as being identical to the original isolated pathogen.
  • Potential Limitations of Koch’s Postulates:     * Unculturable Bacteria: Some agents cannot be grown in pure culture (e.g., obligate intracellular pathogens).     * Subclinical Infection: Individuals may carry the pathogen without showing disease signs.     * Co-infections: Multiple pathogens may be required or present.     * Tumor-related disease: Complex etiologies involving cellular changes.     * Opportunistic Pathogens: May be present in healthy individuals but only cause disease under stress.     * Commensal Pathogens: Naturally present in the microbiome.     * Distant Disease: Signs manifest far from the site of infection.     * Toxin-related disease: Symptoms caused by toxins rather than the presence of the organism itself.