Principles of Infectious Diseases Course Introduction and Core Concepts

Introduction to Principles of Infectious Diseases (PID)

• Course Leadership and Identification     - Course Name: Principles of Infectious Diseases (PID).     - Course Coordinator: Dr. Ricardo Gutiérrez, Lic. Microb., MSc., Ph.D.     - Position: Assistant Professor of Veterinary Bacteriology at Ross University School of Veterinary Medicine (RUSVM).     - Course Context: FALL 2025 semester.

• Significance of Infectious Diseases     - Infectious diseases hold a profound historical impact, having killed more humans than all wars combined throughout the entirety of humankind's history.

• PID Faculty Team     - Immunology: Dr. George Nadas (locum).     - Epidemiology: Dr. Pithua.     - Virology: Dr. Ghosh.     - Parasitology: Dr. Yao.     - Bacteriology and Course Coordination: Dr. Gutiérrez.

• Course Structure and Cumulative Assessment     - The course is organized into blocks, with assessment weightage increasing as the curriculum progresses.     - Cumulative Content Distribution:         - Block 1: Initial focus.         - Block 2: Continued focus.         - Block 3: Includes current material plus $\text{10-25\%}$ from Block 1 and Block 2.         - Block 4: Includes current material plus $\text{10-25\%}$ from previous blocks.         - Block 5: Includes current material plus $\text{10-25\%}$ from previous blocks.         - Block 6: Final evaluation includes current material plus $\text{10-25\%}$ from previous blocks.

• Learning Resources and Recommended Literature     - Primary Textbook: Veterinary Microbiology and Microbial Disease, Second Edition.     - Authors: P.J. Quinn, B.K. Markey, F.C. Leonard, E.S. FitzPatrick, S. Fanning, and P.J. Hartigan.     - Publisher: Wiley-Blackwell.     - Section I Content (Introduction to Microbiology, Infection, Immunity, and Molecular Diagnostic Methods):         - 1: Microbiology, microbial pathogens, and infectious disease.         - 2: Infection and immunity.         - 3: Subdivisions, classification, and morphological characterization of infectious agents.         - 4: Immunodeficiency diseases.         - 5: Vaccines and vaccination.         - 6: Molecular diagnostic methods.     - Section II Content (Introductory Bacteriology):         - 7: The structure of bacterial cells.         - 8: Cultivation, preservation, and inactivation of bacteria.         - 9: Bacterial genetics, mechanisms of genetic variation, and gene databases.         - 10: Laboratory diagnosis of bacterial disease.         - 11: Antibacterial agents.         - 12: Antibacterial resistance.         - 13: Bacterial colonization, tissue invasion, and clinical disease.

• Guidance for Student Success     - Learning objectives are provided per lecture or topic to serve as study guidelines.     - Individual professors present specifics during their respective lectures.     - Recommendations for new students:         - Keep up with daily information (do not fall behind).         - Ask questions frequently.         - Collaborate with peers and Teaching Assistants (TAs).         - Access lecture slides and background information via CANVAS.         - Embrace different teaching styles as a form of academic enrichment.         - Reach out for help via one-on-one meetings, group meetings, or email.

Fundamental Concepts and History of Microbiology

• Definitions and Discovery     - Microorganism (Microbe): Any organism that is invisible to the naked eye.     - First Observation: Antonie van Leeuwenhoek in the 1670s.

• Size Spectrum of Pathogens     - Pathogens range from the molecular level to multicellular organisms:         - Smallest: Prions.         - Intermediate: Viruses.         - Growing size: Bacteria.         - Larger: Fungi.         - Progressively larger: Protozoa.         - Largest: Multicellular parasites (Example: Lengths up to $1\,cm$ or more).

• Historical Figures and Contributions     - Agostino Bassi (1835):         - Demonstrated that a silkworm disease called "muscardine" was contagious.         - Identified the cause as a microscopic fungus: Beauveria bassiana.     - Louis Pasteur (1850s–1880s):         - Researched the role of microbes in fermentation and disease.         - Developed the process of pasteurization.         - Developed vaccines for Rabies and Anthrax.     - Robert Koch (1860s–1880s):         - Identified the specific bacteria responsible for Anthrax, Tuberculosis, and Cholera.         - Developed Koch's Postulates to establish a causal relationship between a microbe and a disease.

Ecological and Biological Classifications

• Ecological Classification     - Microorganisms are classified by their capacity to multiply inside or outside of hosts (animals, humans, arthropods) based on their ecological niche.     - Opportunistic/Facultative: Organisms that can live independently but may cause disease under specific conditions.     - Obligate: Organisms that must reside within a host to survive or reproduce.

• The Microbiome     - Definition: A microbiota constructed from a diverse collection of organisms inhabiting animal tissues.     - Components: Fungi, bacteria, archaea, viruses, and mites.     - Locations: Skin, gastrointestinal tract, respiratory tract, and other tissues.

Terminology of Disease Transmission

• Infectious Disease     - Definition: Disorders caused by organisms such as bacteria, viruses, fungi, or parasites.

• Communicable Disease     - Definition: An infectious disease that spreads from individual to individual.     - Requirements: May require a specific mode of transmission, involving both direct and indirect routes.     - Examples of specific modes: Vector transmission, sexual transmission.

• Contagious Disease     - Definition: A subset of infectious diseases that spread easily from one individual to another.     - Key Transmission Modes: Air (respiratory droplets/aerosols) or physical touch.

• Summary of Distinctions     - Infectious does not always mean transmissible (Example: Tetanus is infectious but not transmissible from person to person).     - Communicable refers to transmissible infectious diseases via various routes.     - Contagious refers specifically to diseases that are easily transmissible via direct contact or close proximity.     - Relationship Hierarchy: All contagious diseases are communicable; all communicable diseases are infectious.     - Negatives: Not all infectious diseases are transmissible; not all communicable diseases are contagious.

• Notifiable and Reportable Diseases     - Definition: Diseases that are legally mandated to be reported to public health authorities to control or prevent spread.     - Terminology usage:         - "Notifiable": Usually refers to jurisdictions at the national level.         - "Reportable": Terminology varies across local, state, and national levels.     - Non-infectious examples: Not all notifiable diseases are infectious (e.g., cancer, lead poisoning, congenital conditions).

• Zoonotic Disease     - Zoonotic Pathogen: A microorganism that exists as a commensal or pathogen on animals but can be transmitted to humans and cause disease.     - Transmission Mechanisms: Vectors (e.g., ticks) or direct contact with the animal or its bioproducts.

Disease Dynamics and Progression

• The Infectious Disease Triad     - Pathogen Factors:         - Pathogenicity.         - Dispersal efficiency.         - Survival efficiency.         - Inoculation dose.     - Environment Factors:         - Abiotic and biotic factors.         - Housing management.         - Population density.         - Sanitation.         - Nutrition.         - Prevention measures.     - Susceptible Host Factors:         - Breed, age, sex, and genotype.         - Physiology and innate susceptibility.         - Immune response.

• Outcomes of Exposure to Infectious Agents     - Exposure does not always lead to infection or disease. Potential paths include:         - No Infection: The pathogen is prevented from colonizing.         - Infection without Disease: The host becomes a carrier or moves to sub-clinical status.         - Infection followed by Disease:             - Recovery: Pathogen elimination and development of immunity or non-immunity.             - Death or Disability.         - Persistence: The host becomes a carrier (active or latent).

• Stages of Disease Progression     - 1. Exposure.     - 2. Incubation/Asymptomatic Period (could lead to becoming a latent carrier).     - 3. Prodromal Period: Characterized by mild, non-specific signs.     - 4. Illness: Full clinical signs and symptoms.     - 5. End of symptoms (Recovery or transition to carrier state).

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 suspected pathogen must be isolated from the diseased organism 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.

• Limitations to Koch’s Postulates     - Several conditions challenge the universality of these rules:         - Unculturable bacteria: Some pathogens cannot be grown in pure culture in a lab.         - Subclinical infection: Pathogens may be present without causing observable disease (violates Postulate 1).         - Co-infections: Disease may be caused by multiple agents acting together.         - Tumor-related disease: Viral-induced cancers may have complex or long-term developments.         - Opportunistic pathogens: Organisms that only cause disease in immunocompromised hosts.         - Commensal pathogens: Organisms that are part of normal flora but can become pathogenic.         - Distant disease: The pathogen is in one location, but symptoms appear elsewhere.         - Toxin-related disease: The disease is caused by a toxin secreted by the bacteria, even if the bacteria themselves are gone or not present at the site of symptoms.