Biology (5090)
Topic 6 of 7Cambridge O Levels

Disease & Immunity

Pathogens, transmission, immune response and vaccinations

Pathogens are microorganisms that cause disease: bacteria (cholera, TB), viruses (COVID, HIV, influenza), fungi (athlete's foot), protoctists (malaria — Plasmodium).


Transmission: Direct contact, droplets (sneezing), contaminated water/food, vectors (mosquitoes carry malaria).


Body defences:

  • Physical barriers: Skin, mucus, stomach acid, tears (lysozyme)
  • White blood cells (immune response):
  • Phagocytes — engulf and digest pathogens (phagocytosis)
  • Lymphocytes — produce antibodies (specific to antigens on pathogen surface)

    • Vaccination: Inject dead/weakened pathogen → body produces antibodies + **memory cells** → faster response if exposed again. **Herd immunity** protects unvaccinated people.


    Antibiotics kill bacteria (NOT viruses). Overuse → antibiotic resistance (MRSA). This is why doctors shouldn't prescribe antibiotics for viral infections like flu.

    Key Points to Remember

    • 1Pathogens: bacteria, viruses, fungi, protoctists
    • 2Phagocytes engulf; lymphocytes make antibodies
    • 3Vaccines create memory cells for faster response
    • 4Antibiotics kill bacteria only, not viruses

    Pakistan Example

    Dengue Fever and EPI — Pakistan's Public Health Battle

    Pakistan's Expanded Programme on Immunisation (EPI) vaccinates millions of children against polio, measles, and hepatitis B. Dengue fever (spread by Aedes mosquitoes) hit Lahore hard in 2011 and 2019 — a vector-borne disease. Karachi's anti-dengue spray campaigns target the vector, not the virus directly.

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