Chapter 13: Viruses, Viroids, and Prions

Overview of Viruses

Viruses are acellular infectious agents that require host cells for replication. They differ from cellular organisms in structure and replication mechanisms.

• Definition: Viruses are obligate intracellular parasites composed of genetic material (DNA or RNA) surrounded by a protein coat.

• Host Range: Each virus infects specific hosts due to receptor compatibility.

• Structure: Viruses consist of a nucleic acid core and a protein coat (capsid). Some have an envelope derived from host membranes.

• Size: Viruses are much smaller than bacteria, typically measured in nanometers.

• Classification: Based on nucleic acid type, capsid symmetry, presence of envelope, and host range.

Viral Multiplication

Viruses replicate by hijacking host cellular machinery. The replication cycle varies between DNA and RNA viruses.

• Lytic Cycle: Virus injects genetic material, replicates, assembles new virions, and lyses host cell.

• Lysogenic Cycle: Viral DNA integrates into host genome, replicates with host, may later enter lytic cycle.

• Example: Bacteriophage lambda exhibits both lytic and lysogenic cycles.

Prions and Viroids

Prions and viroids are non-viral infectious agents.

• Prions: Infectious proteins causing neurodegenerative diseases (e.g., Creutzfeldt-Jakob disease).

• Viroids: Small, circular RNA molecules infecting plants.

Chapter 14 & 15: Principles of Disease, Epidemiology, and Pathogenicity

Definitions and Disease Categories

Understanding disease involves distinguishing between infection, disease, and pathogenicity.

• Infection: Colonization of the body by pathogens.

• Disease: An abnormal state in which the body is not functioning normally.

• Pathogenicity: The ability of a microorganism to cause disease.

• Virulence: Degree of pathogenicity.

Types of Diseases

• Communicable: Can be spread from person to person (e.g., influenza).

• Non-communicable: Not spread between hosts (e.g., tetanus).

• Contagious: Easily spread (e.g., measles).

Patterns of Infection

• Local Infection: Confined to a small area.

• Systemic Infection: Spread throughout the body.

• Focal Infection: Local infection that spreads to other parts.

Stages of Disease

• Incubation Period: Time between exposure and appearance of symptoms.

• Prodromal Period: Early, mild symptoms.

• Period of Illness: Most severe symptoms.

• Period of Decline: Symptoms subside.

• Period of Convalescence: Recovery and return to health.

Transmission of Disease

• Direct Contact: Person-to-person transmission.

• Indirect Contact: Via fomites (inanimate objects).

• Droplet Transmission: Coughing, sneezing.

• Vehicle Transmission: Through water, food, or air.

• Vector Transmission: Via animals (e.g., mosquitoes).

Epidemiology

Epidemiology studies the distribution and determinants of diseases in populations.

• Incidence: Number of new cases in a time period.

• Prevalence: Total number of cases at a given time.

• Mortality Rate: Number of deaths due to a disease.

• Example: CDC tracks incidence and prevalence of infectious diseases.

Microbial Mechanisms of Pathogenicity

• Portals of Entry: Mucous membranes, skin, parenteral route.

• Adherence: Pathogens attach to host tissues via adhesins.

• Penetration/Evasion: Capsules, cell wall components, enzymes.

• Damage to Host: Toxins (exotoxins, endotoxins), direct damage, hypersensitivity reactions.

Chapter 16: Innate Immunity

Overview of Innate Immunity

Innate immunity provides immediate, non-specific defense against pathogens.

• First Line of Defense: Physical and chemical barriers (skin, mucous membranes, secretions).

• Second Line of Defense: Phagocytes, inflammation, fever, antimicrobial proteins.

Phagocytosis

• Definition: Ingestion of microbes or particles by phagocytic cells (e.g., neutrophils, macrophages).

• Steps: Chemotaxis, adherence, ingestion, digestion.

Inflammation

• Purpose: Localizes infection, removes pathogens, repairs tissue.

• Signs: Redness, heat, swelling, pain.

• Cytokines: Chemical messengers that regulate immune responses.

Fever

• Definition: Elevated body temperature as a response to infection.

• Function: Enhances immune activity, inhibits pathogen growth.

Antimicrobial Substances

• Interferons: Proteins that inhibit viral replication.

• Complement System: Group of proteins that enhance immune responses.

Chapter 17: Adaptive Immunity

Overview of Adaptive Immunity

Adaptive immunity is a specific response to pathogens, involving lymphocytes and the production of antibodies.

• Humoral Immunity: Mediated by B cells and antibodies.

• Cell-Mediated Immunity: Mediated by T cells.

Antibody Structure and Function

• Structure: Y-shaped proteins with variable and constant regions.

• Classes: IgG, IgM, IgA, IgD, IgE.

• Function: Neutralization, opsonization, complement activation.

Activation of B and T Cells

• B Cells: Activated by antigen binding and helper T cell signals.

• T Cells: Recognize antigens presented by MHC molecules.

Types of Adaptive Immunity

• Active Immunity: Results from exposure to antigen (natural infection or vaccination).

• Passive Immunity: Transfer of antibodies (e.g., maternal antibodies).

Equations and Formulas

• Incidence Rate:

• Prevalence:

Additional info: Some details, such as specific examples of diseases and immune responses, were expanded for clarity and completeness.