Introduction to Infectious Diseases: Terms, Concepts, and History

Course Overview and Faculty

Institution: Ross University School of Veterinary Medicine (RUSVM).
Course: Principles of Infectious Diseases (PID).
Course Introduction: Infectious diseases have historically killed more humans than all wars combined throughout the entirety of human history.
PID Teaching Team: * Dr. Ricardo Gutierrez (Course Coordinator): Licensed Microbiologist, MSc, Ph.D.; Assistant Professor of Veterinary Bacteriology. * Dr. George Nadas: Specialist in Immunology (locum). * Dr. Pithua: Specialist in Epidemiology. * Dr. Ghosh: Specialist in Virology. * Dr. Yao: Specialist in Parasitology.
Course Structure and Weighting: * The course is divided into six sequential blocks. * Block 1 and Block 2: Foundational material. * Block 3 through Block 6: These blocks carry a cumulative weight, including approximately $10-25\%$ of material from previous blocks to ensure ongoing mastery.

Course Materials and Student Success

Learning Objectives: Objectives are stated per lecture or topic and serve as primary study guidelines. Each professor provides specific details regarding their material during lectures.
Recommended Literature: The primary textbook available on CANVAS is Veterinary Microbiology and Microbial Disease (Second Edition) by P.J. Quinn, B.K. Markey, F.C. Leonard, E.S. FitzPatrick, S. Fanning, and P.J. Hartigan. * Key Sections of the Textbook: 1. Introduction to Microbiology, Infection, Immunity, and Molecular Diagnostic Methods (Chapters 1-6). 2. Introductory Bacteriology (Chapters 7-13), covering cell structure, cultivation, genetics, laboratory diagnosis, antibacterial agents/resistance, and tissue invasion.
Helpful Student Success Tips: * Maintain a consistent schedule to keep up with the high volume of information. * Engage actively by asking questions in class or via email. * Collaborate with peers and utilize Teaching Assistants (TAs). * Prepare for diverse teaching styles from different professors, which is intended to enrich the learning experience. * Availability for one-on-one or group meetings with faculty is provided upon request.

Definition and Classification of Microorganisms

Definition of Microorganism (Microbe): Any organism that is invisible to the naked eye.
Historical Discovery: Microorganisms were first observed by Antonie van Leeuwenhoek in the 1670s.
Size Spectrum (Smallest to Largest): 1. Prions: The smallest infectious agents. 2. Viruses 3. Bacteria 4. Fungi 5. Protozoa 6. Multicellular Parasites: The largest category, which can reach sizes up to and exceeding $1\,cm$ .
Ecological Classification: Microorganisms and larger parasites are classified by their niche and ability to replicate inside or outside of hosts (animals, humans, arthropods): * Opportunistic/Facultative: Organisms that can live independently but may cause disease under certain conditions. * Obligate: Organisms that must reside within a host to complete their life cycle or multiply.

Historical Pioneers of Microbiology

Agostino Bassi (1835): Demonstrated that muscardine, a disease in silkworms, was contagious and caused by the microscopic fungus Beauveria bassiana.
Louis Pasteur (1850s–1880s): * Investigated the role of microbes in fermentation and disease. * Developed the process of pasteurization to kill harmful microbes in food/liquids. * Developed vaccines for rabies and anthrax.
Robert Koch (1860s–1880s): * Identified the specific bacteria responsible for anthrax, tuberculosis, and cholera. * Developed "Koch's Postulates," a rigorous set of criteria to establish the causative link between a microbe and a disease.

Key Terminology in Infectious Diseases

Microbiome: A complex community (microbiota) consisting of fungi, bacteria, archaea, viruses, and mites that covers various animal tissues, including the skin, gastrointestinal tract, and respiratory tract.
Infectious Disease: Disorders caused by pathogenic organisms such as bacteria, viruses, fungi, or parasites.
Communicable Disease: Any infectious disease that can spread from one individual to another. Transmission can occur via direct or indirect routes and may require specific modes (e.g., sexual transmission or vector transmission).
Contagious Disease: A subset of communicable diseases that spread easily between individuals through physical touch or air (close proximity).
Hierarchical Relationships of Disease Terms: * Infectious ≠ transmissible: Not all infectious diseases spread between hosts (e.g., Tetanus). * Communicable = Transmissible: All communicable diseases are infectious, but not all infectious diseases are communicable. * Contagious = Easily Transmissible: All contagious diseases are communicable, but not all communicable diseases are contagious enough to be considered "contagious."

Notifiable and Zoonotic Diseases

Notifiable (Reportable) Disease: Diseases that are legally mandated to be reported to public health authorities to control or prevent spread. * The term "notifiable" typically implies national jurisdiction. * The term "reportable" may vary across local, state, or national levels. * Note: Not all notifiable diseases are infectious (e.g., cancer, lead poisoning, or congenital conditions may be notifiable in certain jurisdictions).
Zoonotic Disease: Caused by zoonotic pathogens, which are microorganisms that act as commensals or pathogens in animals but can be transmitted to humans to cause disease. * Modes of Transmission: Vector-borne (e.g., ticks) or direct contact with the animal or its bioproducts.

The Infectious Disease Triad

The existence and severity of an infectious disease depend on the interaction of three factors:

Pathogen Factors: * Pathogenicity (ability to cause disease). * Dispersal efficiency. * Survival efficiency in the environment. * Inoculation dose (the amount of pathogen required to start an infection).
Susceptible Host Factors: * Demographics: Breed, age, sex, and genotype. * Physiology: Individual susceptibility and current health status. * Immune Response: Effectiveness of the host's defense mechanisms.
Environment Factors: * Abiotic and biotic factors. * Housing management and population density. * Sanitation and nutrition levels. * Prevention strategies (e.g., biosecurity).

Exposure Outcomes and Stages of Infection

Exposure vs. Infection: Exposure to an infectious agent does not always result in infection.
Possible Outcomes of Exposure: * No Infection: Pathogen fails to colonize. * Infection with Subclinical Disease: The host is infected but shows no clinical signs; may become a commensal-like relationship (carrier state). * Infection with Clinical Disease: The host shows symptoms, potentially leading to: 1. Pathogen Elimination and Immunity: The host recovers and develops resistance. 2. Pathogen Elimination and Non-immunity: The host recovers but remains susceptible. 3. Carrier State (Persistence): The host survives but continues to shed the pathogen (Active vs. Latent carriers). 4. Death or Disability.
Stages of Infectious Disease: * Exposure → Prodromal Stage (early, mild signs) → Illness (peak symptoms) → End of symptoms (Recovery or Latency).

Koch’s Postulates and Their Limitations

The Four Postulates (1884): 1. The suspected pathogen must be present in every case of the disease and absent from healthy individuals. 2. The pathogen must be isolated from the diseased host and grown in a pure culture. 3. The cultured pathogen must cause the same disease when inoculated into a healthy, susceptible host. 4. The same pathogen must be re-isolated from the newly diseased host.
Modern Limitations of Koch’s Postulates: * Unculturable Bacteria: Many organisms cannot be grown in pure culture in a laboratory. * Subclinical Infection: Postulate 1 fails if healthy individuals carry the pathogen without signs. * Co-infections: Multiple pathogens may be required to cause a specific disease. * Opportunistic/Commensal Pathogens: Some microbes only cause disease when the host's environment or immunity changes. * Other complexities: Tumor-related diseases, toxin-related diseases (where the bacteria may no longer be present), and distant diseases.