Principles of Disease and Epidemiology

Key Definitions and Relationships

• Pathology: The scientific study of disease, including its causes, development, and effects on the body.

• Etiology: The study of the cause of a disease.

• Infection: The invasion and multiplication of microorganisms in body tissues, which may or may not cause disease.

• Relationship: Etiology identifies the cause, pathology studies the effects, and infection is the process that may lead to disease.

Normal vs. Transient Microbiota

• Normal microbiota: Microorganisms that permanently colonize the host without causing disease under normal conditions.

• Transient microbiota: Microbes present in the body for a short time without causing disease.

Protection by Normal Microbiota

• Microbial antagonism: Normal microbiota compete with pathogens for nutrients and space, producing substances harmful to invading microbes.

• Competitive exclusion: Normal microbiota outcompete potential pathogens, reducing their ability to colonize the host.

Opportunistic Pathogens

• Opportunistic pathogens: Normally harmless microbes that cause disease when the host's defenses are compromised or when introduced to unusual body sites.

• Example: Escherichia coli causing urinary tract infections when entering the urinary tract.

Koch’s Postulates

• Set of criteria used to prove that a specific microorganism causes a specific disease.

• Steps:

  1. The microorganism must be found in all cases of the disease.

  2. It must be isolated and grown in pure culture.

  3. The cultured microbe must cause disease when introduced into a healthy host.

  4. It must be re-isolated from the experimentally infected host.

• Significance: Foundation for identifying disease-causing agents.

Types of Diseases

• Communicable disease: Can be spread from one host to another (e.g., influenza).

• Contagious disease: Easily spread from person to person (e.g., measles).

• Noncommunicable disease: Not spread from host to host (e.g., tetanus).

Occurrence of Disease

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

• Prevalence: Total number of cases (new and existing) at a given time.

• Sporadic: Occurs occasionally (e.g., typhoid fever).

• Endemic: Constantly present in a population (e.g., common cold).

• Epidemic: Many people acquire a disease in a short time (e.g., influenza outbreak).

• Pandemic: Worldwide epidemic (e.g., COVID-19).

Classification of Diseases

• Acute: Develops rapidly, short duration (e.g., influenza).

• Chronic: Develops slowly, persists (e.g., tuberculosis).

• Latent: Pathogen inactive for a time, then activates (e.g., herpes simplex).

• Subclinical: No noticeable symptoms (e.g., asymptomatic polio).

Stages of Disease

• Incubation period: Interval between infection and first symptoms.

• Prodromal period: Early, mild symptoms.

• Period of illness: Most severe signs and symptoms.

• Period of decline: Signs and symptoms subside.

• Period of convalescence: Recovery; body returns to pre-disease state.

Transmission of Disease

• Contact transmission: Direct (person-to-person), indirect (via fomites), or droplet (coughing, sneezing).

• Vehicle transmission: Via medium such as water, food, or air.

• Vector transmission: Via animals (usually arthropods) that carry pathogens (mechanical or biological transmission).

Nosocomial (Healthcare-Associated) Infections

• Acquired in healthcare settings.

• Common causes: Compromised host, chain of transmission, presence of microorganisms in hospital environment.

• Examples: Urinary tract infections, surgical site infections, pneumonia.

Microbial Mechanisms of Pathogenicity

Pathogenicity and Virulence

• Pathogenicity: Ability of a microorganism to cause disease.

• Virulence: Degree of pathogenicity; how severe the disease is.

Portals of Entry

• Microbes enter the host via specific routes: mucous membranes, skin, parenteral route (punctures, bites, wounds).

Adherence Factors

• Adhesins/ligands: Surface molecules on pathogens that bind to host cell receptors, enabling colonization.

• Example: Fimbriae of Escherichia coli binding to urinary tract epithelium.

Evasion of Host Defenses

• Capsules: Prevent phagocytosis.

• Enzymes: E.g., coagulase (clots blood), hyaluronidase (breaks down connective tissue).

• Antigenic variation: Pathogens alter surface antigens to evade immune response.

Damage to Host Cells

• Direct damage: Pathogens disrupt host cell function, produce waste products, or multiply inside host cells.

• Indirect damage: Toxins or immune responses cause tissue damage.

Exotoxins vs. Endotoxins

• Exotoxins: Proteins secreted by bacteria; highly toxic, specific effects.

• Endotoxins: Lipid A component of Gram-negative bacterial outer membrane; released upon cell death, cause generalized effects (e.g., fever, shock).

Major Types of Exotoxins

• A-B toxins: Two-part toxins (active and binding components).

• Membrane-disrupting toxins: Cause cell lysis.

• Superantigens: Cause intense immune response.

Pathogenicity of Fungi, Protozoa, Helminths, and Algae

• Fungi: Produce toxins (mycotoxins), enzymes, or provoke allergic responses.

• Protozoa: Cause damage by growing in host tissues and producing waste products.

• Helminths: Use host nutrients, cause tissue damage, or produce toxic waste.

• Algae: Some produce neurotoxins (e.g., dinoflagellates).

Host Immune Response

• Essential for controlling and eliminating pathogens.

• Overactive or inappropriate responses can contribute to disease.

Innate Immunity: Nonspecific Defenses of the Host

Innate vs. Adaptive Immunity

• Innate immunity: Present at birth, nonspecific, immediate response.

• Adaptive immunity: Specific, acquired over time, involves memory.

First Line of Defense

• Physical factors: Skin, mucous membranes, cilia, tears, saliva.

• Chemical factors: Lysozyme, gastric juice, low pH, sebum.

• Microbial factors: Normal microbiota compete with pathogens.

Role of Normal Microbiota

• Compete with pathogens, stimulate immune system, produce antimicrobial substances.

Second Line of Defense

• Phagocytosis: Ingestion and destruction of microbes by phagocytes (e.g., neutrophils, macrophages).

• Inflammation: Local response to injury or infection; redness, heat, swelling, pain.

• Fever: Systemic response; elevated body temperature inhibits pathogens and enhances immune response.

• Antimicrobial substances: Complement system, interferons, transferrins.

Phagocytosis Process and Evasion

• Steps: Chemotaxis, adherence, ingestion, digestion.

• Pathogens evade by capsules, inhibiting phagosome-lysosome fusion, or surviving inside phagocytes.

Stages and Purpose of Inflammation

• Vasodilation, increased permeability, migration of phagocytes, tissue repair.

• Purpose: Contain infection, remove pathogens, initiate repair.

Fever in Defense

• Induced by pyrogens (e.g., cytokines, endotoxins).

• Inhibits microbial growth, increases metabolism, enhances immune response.

Interaction with Adaptive Immunity

• Innate responses activate and shape adaptive immunity (e.g., antigen presentation by dendritic cells).

Adaptive Immunity: Specific Defenses of the Host

Humoral vs. Cellular Immunity

• Humoral immunity: Mediated by B cells and antibodies; targets extracellular pathogens.

• Cellular immunity: Mediated by T cells; targets intracellular pathogens and abnormal cells.

Key Terms

• Antigen: Substance that provokes an immune response.

• Epitope: Specific region on antigen recognized by immune cells.

• Hapten: Small molecule that is antigenic only when attached to a carrier.

Antibodies (Immunoglobulins)

• Y-shaped proteins produced by B cells; bind specific antigens.

• Structure: Two heavy chains, two light chains, variable (antigen-binding) and constant regions.

Classes of Antibodies

B Cell Activation and Antibody Production

• B cells recognize antigen, are activated (often with T helper cell help), and differentiate into plasma cells that secrete antibodies.

T Cell Functions

• T helper cells (CD4+): Activate B cells, other T cells, and macrophages.

• Cytotoxic T cells (CD8+): Kill infected or abnormal cells.

• Regulatory T cells: Suppress immune responses to prevent autoimmunity.

Types of Adaptive Immunity

Immunological Memory

• Primary response: First exposure, slower, lower antibody levels.

• Secondary response: Subsequent exposures, faster, higher antibody levels due to memory cells.

Applications: Vaccines and Immunotherapy

• Vaccines stimulate adaptive immunity and memory without causing disease.

• Immunotherapy uses antibodies or immune cells to treat disease.

Disorders Associated with the Immune System

Hypersensitivity Reactions

• Hypersensitivity: Excessive or inappropriate immune response to antigens.

• Immediate (Types I, II, III): Occur within minutes to hours.

• Delayed (Type IV): Occurs after 24–48 hours.

Types of Hypersensitivity

Anaphylactic Reactions

• Systemic: Life-threatening, e.g., shock, airway obstruction.

• Localized: Limited to specific tissues, e.g., hay fever, asthma.

Cytotoxic Hypersensitivity

• Antibodies target cells, leading to cell destruction via complement.

• Examples: Blood transfusion reactions, hemolytic disease of the newborn.

Immune Complex Hypersensitivity

• Antigen-antibody complexes deposit in tissues, causing inflammation and tissue damage.

• Examples: Glomerulonephritis, serum sickness.

Delayed Cell-Mediated Hypersensitivity

• Mediated by T cells; symptoms appear after 24–48 hours.

• Examples: Contact dermatitis (poison ivy), tuberculosis skin test.

Autoimmune Diseases

• Immune system attacks self-antigens due to loss of self-tolerance or cross-reactive antibodies.

• Examples: Rheumatoid arthritis, lupus, Graves’ disease, multiple sclerosis.

Transplantation Immunology

• Transplant rejection: Immune response against foreign antigens on graft.

• Immunosuppressive therapy: Drugs used to prevent rejection by suppressing immune response.

Immune Response to Cancer

• Immune system can recognize and destroy tumor cells (cancer surveillance).

• Tumors may evade immunity by antigenic variation or suppressing immune responses.

Immunodeficiency

• Immunodeficiency: Failure of the immune system to protect against disease.

• Congenital (primary): Genetic defects present at birth.

• Acquired (secondary): Result from infections (e.g., HIV), drugs, or other factors.

HIV Infection and AIDS

• HIV infects and destroys CD4+ T cells, leading to immunodeficiency (AIDS).

• Stages of HIV infection:

  1. Acute infection: Flu-like symptoms, high viral load.

  2. Clinical latency: Asymptomatic, slow decline of CD4+ cells.

  3. AIDS: Severe immunodeficiency, opportunistic infections, cancers.

• Diagnosis: Detection of antibodies (ELISA, Western blot), viral RNA (PCR).

• Treatment: Antiretroviral therapy (ART) suppresses viral replication.

• Prevention: Safe sex, needle exchange, screening blood products, pre-exposure prophylaxis (PrEP).

Additional info: These notes are structured to cover the main learning objectives for exam preparation in college-level microbiology, focusing on disease principles, pathogenicity, immunity, and immune disorders.