Core Concepts in Infection 

1. Pathogenesis of Infection

•    Steps of infection:

o    Colonisation: microorganism presence without tissue invasion

o    Invasion: entry and multiplication within host tissues

o    Evasion: avoiding host immune responses

o    Dissemination: local spread or haematogenous dissemination

•    Virulence factors:

o    Toxins:

    Exotoxins (e.g., tetanus, diphtheria)

    Endotoxins (LPS in Gram-negative bacteria → sepsis)

o    Adhesion proteins:

    Pili, fimbriae (e.g., E. coli UTI)

o    Capsules:

    Avoid phagocytosis (e.g., S. pneumoniae, Neisseria meningitidis)

o    Biofilm formation:

    E.g., Staphylococcus epidermidis on catheters, prosthetic devices

2. Modes of Transmission

Direct contact

Herpes simplex virus (HSV), HIV, MRSA

Droplet

Influenza, SARS-CoV-2, Neisseria meningitidis

Airborne

Tuberculosis, measles, varicella-zoster

Faeco-oral

Cholera, hepatitis A, norovirus, poliovirus

Vector-borne

Malaria (Anopheles), dengue, Zika, Lyme disease (ticks), leishmaniasis (sandflies)

Zoonotic

Leptospirosis (rats), brucellosis (unpasteurised dairy), rabies (dogs, bats), Q fever (Coxiella burnetii, livestock)

•    Fomite transmission: indirect via contaminated surfaces (e.g., RSV, norovirus)

3. Herd Immunity & R₀ (Basic Reproductive Number)

•    R₀ (basic reproductive number):

o    Average number of secondary cases from one index case in a fully susceptible population

•    Interpretation:

o    R₀ >1: infection can spread in the population

o    R₀ <1: infection likely to die out

•    Herd immunity threshold:

o    Formula: 1 – (1/R₀)

o    Examples:

    Measles: R₀ ≈ 12–18 → ~95% needed for herd immunity

    COVID-19 original strain: R₀ ≈ 2–3 → ~60–70% threshold

4. Healthcare-Associated Infections (HAIs)

•    Common HAIs:

o    MRSA: surgical site, bloodstream infections

o    Clostridioides difficile: antibiotic-associated colitis

o    Catheter-associated urinary tract infections (CAUTI)

o    Central line–associated bloodstream infections (CLABSI)

o    Ventilator-associated pneumonia (VAP)

•    Key control measures:

o    Strict hand hygiene (alcohol-based rub or soap and water for C. difficile)

o    Barrier precautions (gloves, gowns)

o    Environmental cleaning

o    Judicious catheter/device use; early removal when possible

•    Antimicrobial stewardship:

o    Reduces selective pressure and multi-drug resistant organisms (MDROs)

5. Vaccination

•    Live attenuated vaccines:

o    Examples: MMR (measles, mumps, rubella), yellow fever, varicella, intranasal influenza, BCG

o    Contraindicated in severe immunosuppression (e.g., HIV with CD4 <200, chemotherapy, high-dose steroids)

•    Inactivated (killed) vaccines:

o    Examples: injectable influenza, hepatitis A, rabies, HPV

o    Safe in immunocompromised patients but may be less immunogenic

•    Toxoid vaccines:

o    Examples: tetanus, diphtheria

•    Subunit/conjugate vaccines:

o    Examples: pneumococcal (PCV13, PPV23), meningococcal, Hib

Extra Revision Pearls

•    BCG (Mycobacterium bovis strain):

o    Given at birth in high-risk groups in UK

o    Protects against severe TB forms (e.g., TB meningitis)

•    Post-exposure prophylaxis examples:

o    HIV: antiretrovirals within 72 hours

o    Hepatitis B: HBV vaccine ± HBIG

o    Rabies: vaccine + rabies immunoglobulin

•    Needlestick injury protocol:

o    Immediate washing

o    Baseline serology (HIV, HBV, HCV)

o    Consider PEP