BIOS 2210 Chapter 15/16

Infection

  • Invasion and multiplication of a microbe in the body, which may or may not cause symptoms.

Disease

  • When infection disrupts normal body function, causing symptoms and signs.

Symptoms

  • Subjective effects felt by the patient (e.g., pain, fatigue, nausea).

Transmissible Disease

  • A disease that can spread from person to person (also called communicable).

Epidemiology

  • The study of how diseases spread, their patterns, causes, and control in populations.

Pathogen

  • A microbe that can cause disease.

Opportunistic Pathogen

  • Normally harmless, but can cause disease in weakened immune systems or disrupted microbiota.

Pathogenicity

  • The ability of a microbe to cause disease.

Virulence

  • The degree of harm a pathogen causes; how severe the disease is.

Virulence Factor

  • Specific traits (toxins, enzymes, etc.) that help a pathogen invade, evade, or damage the host.

Modes of Transmission:

  1. Direct Contact – physical touch, droplets, sexual contact

  2. Indirect Contact – touching contaminated surfaces (fomites)

  3. Airborne – inhaling aerosols or fine droplets

  4. Vector-borne – via insects like mosquitoes or ticks

  5. Vehicle transmission – through food, water, or air

  6. Vertical – mother to child during pregnancy or birth

Prodromal Stage of Disease

  • Early stage with mild, non-specific symptoms (fatigue, headache) before the full illness.

Relationship Between Pathogen Load & Symptoms

  • As the number of pathogens increases, symptoms usually become more severe.

  • Often shown on a graph: pathogen number rises first, then symptom severity follows.

  • What is bacteremia and septicemia?

    • Bacteremia: The presence of bacteria in the bloodstream. It may be temporary (like after brushing your teeth) or potentially serious if not cleared quickly by the immune system.

    • Septicemia (also called sepsis): A serious, life-threatening condition caused by the multiplication of pathogens (like bacteria) in the blood, triggering an overwhelming immune response. It can lead to septic shock, organ failure, and death if untreated.

    Bacterial Virulence Factors

    Virulence factors help bacteria invade the host, cause disease, and evade immune defenses.

    Adhesins:

    • Surface proteins or molecules that allow bacteria to stick to host cells (like Velcro).

    • Critical for colonization; without adhesins, many pathogens can’t infect.

    • Example: E. coli has fimbriae that attach to urinary tract epithelial cells.

    Exoenzymes (like collagenase):

    • Enzymes secreted by bacteria to break down host tissues and spread the infection.

    • Collagenase: Breaks down collagen, a key structural protein in connective tissue.

      • Allows bacteria to invade deeper tissues.

      • Example: Clostridium perfringens produces collagenase (causes gas gangrene).

    Exotoxins:

    • Proteins secreted by bacteria that are toxic to host cells.

    • Often very potent and specific in their effects.

    • Examples:

      • Clostridium botulinum – botulinum toxin (causes flaccid paralysis).

      • Corynebacterium diphtheriae – diphtheria toxin.

      • Staphylococcus aureus – enterotoxins (cause food poisoning).

    Endotoxins:

    • Part of the outer membrane of Gram-negative bacteria (specifically the lipid A portion of lipopolysaccharide or LPS).

    • Released when the bacteria die and the cell wall breaks apart.

    • Causes inflammation, fever, and in high doses, septic shock.

    • Example: Escherichia coli and Salmonella species (all Gram-negative bacteria).

    What is an intracellular pathogen?

    • A pathogen that lives and replicates inside host cells.

    • Hides from the immune system by being “inside” cells.

    • Can be obligate intracellular (must be inside cells) or facultative (can survive inside or outside).

    • Example:

      • Listeria monocytogenes – facultative intracellular.

      • Chlamydia trachomatis – obligate intracellular.

    Factors That Increase Viral Virulence

    Virulence in viruses is influenced by their ability to:

    1. Evade immune responses:

      • Some viruses block antigen presentation (e.g., Herpesvirus).

    2. High replication rate:

      • Increases damage to host tissues.

    3. Mutability:

      • Like influenza virus (especially RNA viruses), which frequently mutates and escapes immunity.

    4. Tissue tropism:

      • Ability to infect specific cell types that are critical (e.g., HIV infects CD4+ T-cells).

    5. Immune suppression:

      • HIV and measles suppress the immune system, making secondary infections more likely.

    Who is the WHO?

    • WHO = World Health Organization.

    • A United Nations agency that coordinates global health efforts, monitors disease outbreaks, and sets international health standards.

    • Helps with:

      • Disease prevention programs.

      • Immunization campaigns.

      • Emergency responses to pandemics.

      • Promoting public health initiatives globally.