The big idea: An infectious disease is an illness caused by an organism (or particle) that gets into the body and harms it.
The organism that causes the disease is called a pathogen.
Pathogens come in four main types — bacteria, viruses, fungi and protists — and they cause disease by damaging cells or by releasing toxins (poisons).
The body's first job is to stop pathogens getting in at all — that is the role of the primary (first-line) defences: the skin, mucous membranes and stomach acid.
| Type of pathogen | What it is | Example disease |
|---|---|---|
| Bacterium | A single-celled prokaryote; some make toxins that damage the body | Cholera, tuberculosis |
| Virus | A tiny non-living particle that hijacks host cells to copy itself | Influenza (flu), COVID-19 |
| Fungus | A eukaryote (often a mould or yeast) that grows on or in tissues | Athlete's foot, thrush |
| Protist | A single-celled eukaryote, often spread by another organism | Malaria |
- Pathogen
- An organism or particle that causes disease — for example a bacterium, virus, fungus or protist.
- Infectious disease
- A disease caused by a pathogen, which can be passed from one organism to another.
- Toxin
- A poisonous substance released by some pathogens (especially bacteria) that damages the body's cells or tissues.
- Primary (first-line) defence
- A barrier that stops pathogens entering the body in the first place — the skin, mucous membranes and stomach acid.
- Mucous membrane
- A moist lining of body openings (airways, gut) that produces sticky mucus to trap pathogens.
Two ways pathogens harm us: Pathogens make us ill in two main ways:
they damage the cells they infect (for example a virus bursting a host cell), or they release toxins that disrupt how cells work.
Many of the symptoms of an infection — fever, diarrhoea, pain — come from this damage and from the body's response to it.
Before any pathogen can cause disease, it has to get inside the body.
The primary defences are the barriers that try to stop this happening. They are non-specific — they work against any pathogen, not just one kind — and they are the body's first line of defence.
Skin — the physical wall: The skin is a tough, dry physical barrier made of layers of cells, with a surface of dead cells that pathogens cannot easily cross.
As long as the skin is unbroken, it keeps almost all pathogens outside the body.
This is why a cut, wound or burn is dangerous — it breaks the barrier and gives pathogens a way in.
Mucous membranes — trap and sweep: Openings like the airways and gut are lined with mucous membranes that cannot be covered by skin.
These linings make sticky mucus that traps pathogens in inhaled air or food.
In the airways, tiny hair-like cilia then sweep the mucus (and the trapped pathogens) up and away from the lungs, where it is swallowed or coughed out.
Stomach acid — the chemical barrier: Pathogens swallowed in food, drink or mucus reach the stomach.
The stomach contains strong hydrochloric acid, giving it a very low pH (very acidic).
These acidic conditions kill most pathogens before they can reach the intestines — so stomach acid is a chemical primary defence, not a physical one.
| Primary defence | Where it is | How it blocks pathogens |
|---|---|---|
| Skin | The outer surface of the body | A tough, dry physical barrier of dead cells that pathogens cannot easily pass through |
| Mucous membranes | Lining the airways, gut and other openings | Sticky mucus traps pathogens; in the airways, cilia sweep the trapped mucus away |
| Stomach acid | Inside the stomach | Strongly acidic conditions kill most pathogens that are swallowed in food or mucus |
Physical barriers
- Skin — a dry wall of dead cells
- Mucus — traps pathogens in airways and gut
- Cilia — sweep trapped mucus away from the lungs
- Work by physically blocking entry
Chemical barrier
- Stomach acid — strong acid, very low pH
- Kills most swallowed pathogens
- Protects the gut from infection
- Works by chemically destroying pathogens
Why this is exam gold: Examiners love to test the primary defences by removing one of them.
If the skin is cut, pathogens enter through the wound.
If the cilia are damaged (for example by smoking), mucus and pathogens stay in the lungs.
If stomach acid is reduced (for example by acid-lowering medicine), more swallowed pathogens survive and cause infection.
In every case, weakening a barrier means more infection — the cause and effect is the whole point.
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How this is tested: These questions usually arrive as a short data / scenario on Paper 3, built around one infection.
A 3-mark Explain question asks how a named pathogen causes a symptom — link the pathogen to the tissue it affects, then to the symptom (cause → effect → effect).
A 1-mark State question asks how the disease can lead to death — usually a direct consequence of the main symptom (such as fluid loss).
A 2-mark Suggest question gives a scenario that weakens a primary defence (for example a patient on acid-reducing medicine) and asks why infection becomes more likely.
IB-style question — explain how a gut bacterium causes diarrhoea
A toxin-producing bacterium infects the lining of the small intestine. Explain how this bacterium causes watery diarrhoea. [3]
How to score all three marks
- Start with the toxin. The bacterium releases a toxin that acts on the cells lining the intestine.
- State the effect on the cells. The toxin makes these lining cells secrete ions (salts) and water into the gut, instead of absorbing them.
- Link to the symptom. Because so much water passes into the intestine, the gut contents become very watery, producing watery diarrhoea. (Mark 1: toxin released. Mark 2: lining cells secrete water/ions into the gut. Mark 3: this watery fluid leaves as diarrhoea.)
Final answer
The bacterium releases a toxin; the toxin makes the intestinal lining cells secrete ions and water into the gut; the extra water in the intestine is lost as watery diarrhoea.
✓ Why this scores full marks: It follows a clear cause → effect → symptom chain: toxin → cells secrete water → watery diarrhoea.
An Explain worth 3 marks needs each linked step, not just the word 'toxin' or just the symptom on its own.
The same scenario continues with a 1-mark follow-up:
IB-style question — state how the disease can cause death
State how severe watery diarrhoea can lead to death if it is not treated. [1]
Model answer
- Name the direct consequence. So much water (and salts) is lost from the body in the diarrhoea that the person becomes severely dehydrated, which can be fatal. (1 mark: dehydration / severe loss of body fluid / water.)
Final answer
Dehydration — the large loss of water (and salts) from the body can be fatal.
| Primary defence | Where it is | How it blocks pathogens |
|---|---|---|
| Skin | The outer surface of the body | A tough, dry physical barrier of dead cells that pathogens cannot easily pass through |
| Mucous membranes | Lining the airways, gut and other openings | Sticky mucus traps pathogens; in the airways, cilia sweep the trapped mucus away |
| Stomach acid | Inside the stomach | Strongly acidic conditions kill most pathogens that are swallowed in food or mucus |