Cambridge IGCSE Biology
Diseases & Immunity
Your body defends itself against pathogens through three layers of protection, and when it wins a battle, it remembers how—that's immunity.
Quick Overview
- Pathogens & Barriers: Disease-causing organisms spread via direct or indirect contact; your body blocks them with mechanical, chemical, and cellular defences.
- Controlling Spread: Clean water, hygiene, sanitation, and waste disposal prevent pathogens from spreading to new hosts.
- Active Immunity: Your body makes antibodies and memory cells after infection or vaccination—slow to start, lasts for life.
- Antigens & Antibodies: Lymphocytes recognize foreign antigens and produce complementary antibodies that agglutinate pathogens and signal for destruction.
- Vaccination: Dead or weakened pathogens trigger immune response without causing illness, creating long-lasting immunity.
- Herd Immunity: When enough people are vaccinated, the pathogen has nowhere to breed—everyone (including unvaccinated) is protected.
- Passive Immunity: Antibodies received from mother via breast milk—fast-acting but temporary.
- Cholera: Bacterial toxin forces intestinal cells to leak chloride ions; water follows by osmosis, causing severe dehydration.
Pathogens & Barriers
What is a Pathogen?
A pathogen is any disease-causing organism. Most pathogens are bacteria, viruses, or fungi. The key point: pathogens are transmitted from one host to another, which is why we call the diseases they cause transmissible diseases.
Pathogen
A disease-causing organism (bacterium, virus, fungus, or protozoan) that is passed from one host to another, making the diseases they cause transmissible.
How Pathogens Spread
Pathogens don't just appear in your body—they get there. Understanding transmission is crucial because it tells us how to stop them.
| Transmission Method |
How It Works |
Example Diseases |
| Direct contact |
Pathogen passed directly from one host to another via body fluids (blood, semen, saliva) |
HIV, gonorrhoea, hepatitis B, hepatitis C |
| Droplets in air |
Pathogen leaves host as tiny droplets when breathing, coughing, or sneezing; inhaled by another person |
Common cold, influenza |
| Food or water |
Pathogen contaminates food or drinking water; ingested by new host |
Cholera, typhoid, dysentery |
| Contaminated surfaces |
Pathogen survives on surfaces; transferred via touch to skin or mucous membranes |
Athlete's foot, salmonella (food poisoning) |
| Insect vectors |
Insect (mosquito, tick) carries pathogen from infected person and injects it into new host |
Malaria, dengue fever, Lyme disease |
Why does this matter? Once you know how a disease spreads, you can stop it. Cholera spreads via water? Build sewage systems. Flu spreads via droplets? Tell people to cover coughs.
The Body's Three-Layer Defence
Your body doesn't just hope pathogens don't enter—it has three layers of active defence. Imagine them as a castle: walls, guards, and an army.
Layer 1: Mechanical Barriers
hard for pathogens to get in