Question 1
Paper 4 short-answer style1 markState what is meant by a pathogen. (1 mark)
Model answer
A pathogen is a disease-causing organism.
Why this scores
One mark for 'disease-causing organism / microorganism'.
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Detailed notes on Diseases and Immunity for Cambridge IGCSE Biology, covering key concepts, explanations, examples, and exam-focused revision points.
What pathogens are, how they spread, and the body's first line of defence — physical and chemical barriers.
Mapped to the Cambridge IGCSE 0610 syllabus (2026-2028).
Bacteria, viruses, fungi, protoctists. Each acts differently inside the body.
1. Bacteria.
2. Viruses.
3. Fungi.
4. Protoctists.
Worked qualitative. Why don't antibiotics work against viruses?
Cambridge tip. Always classify the pathogen by its FOUR-type group. 'Microbe' or 'germ' is too vague.
Direct contact, air, water/food, vectors. Each route demands different prevention.
1. Direct contact.
2. Air (droplets).
3. Water and food.
4. Vectors.
Worked qualitative. Why was clean drinking water in 19th-century London so important?
Cambridge tip. Cambridge often gives a scenario and asks students to identify the route. Read carefully — 'sneezed near someone' = air; 'shared a needle' = direct/blood; 'mosquito bite' = vector.
Skin, mucus + cilia, stomach acid, tears + lysozyme. Stop pathogens entering in the first place.
Before the immune system kicks in, the body has a series of physical and chemical barriers.
1. Skin (physical).
2. Mucus and cilia (airways).
3. Stomach acid (chemical).
4. Tears, saliva, sweat (lysozyme).
5. Stomach + gut barrier in addition to acid:
Worked qualitative. Why does smoking cause more lung infections?
Cambridge tip. Cambridge marks the MECHANISM, not just the part. Don't say 'tears clean eyes' — say 'tears contain lysozyme which breaks bacterial cell walls'.
Verbatim phrases and definitions Cambridge mark schemes credit.
Diseases appear Paper 2 (3-5 marks: pathogen types + transmission) and Paper 4 (5-7 marks: defences in detail). Examiner reports flag students calling viruses 'cells', confusing vector with pathogen, and listing defences without mechanisms.
Sources: Cambridge IGCSE Biology 0610 syllabus 2026-2028 (10.1); 0610/22 May/Jun 2024 — Q8 (diseases); 0610 Examiner Reports 2022-2024. Last reviewed 2026-05-07.
Step-by-step solutions to past-paper-style questions on diseases , written exactly the way a tutor would explain them at the board.
Question
State what is meant by a pathogen and by a transmissible disease, and give an example of a pathogen.
Step-by-step solution
Step 1
A pathogen is a disease-causing organism (for example a bacterium, virus, fungus or protoctist).
Step 2
A transmissible disease is one in which the pathogen can be passed from one host to another.
Step 3
Example: the bacterium that causes cholera, or the virus that causes influenza (flu).
Answer
A pathogen is a disease-causing organism; a transmissible disease can be passed from one host to another (e.g. cholera, caused by a bacterium).
Question
Describe how a pathogen can be transmitted by (a) direct contact and (b) indirect contact.
Step-by-step solution
Step 1
(a) Direct contact: the pathogen passes straight from an infected person to a healthy one — for example through blood or other body fluids (such as HIV).
Step 2
(b) Indirect contact: the pathogen reaches a new host without direct touch — for example through contaminated food or water (cholera), through the air in droplets from coughs and sneezes (influenza), or carried by an animal/vector (malaria via mosquitoes).
Answer
Direct: through blood or body fluids. Indirect: through contaminated food/water, through the air in droplets, or carried by an animal (vector).
Question
Describe how the skin, mucus, cilia and stomach acid help to prevent pathogens entering or surviving in the body.
Step-by-step solution
Step 1
Skin: a tough physical barrier of cells covering the body, keeping pathogens out.
Step 2
Mucus in the airways: a sticky layer that traps pathogens and dust in the air breathed in.
Step 3
Cilia: tiny hair-like structures on the cells lining the airways that sweep the trapped mucus up to the throat to be swallowed.
Step 4
Stomach acid: the hydrochloric acid in the stomach kills most pathogens that are swallowed in food and drink.
Answer
Skin (physical barrier), mucus (traps pathogens), cilia (sweep mucus away), stomach acid (kills swallowed pathogens).
Examiner tip
Each defence must be paired with HOW it works. 'Skin keeps germs out' earns 0; 'skin is a physical barrier' earns the mark.
Question
Describe four ways the spread of a transmissible disease such as cholera can be controlled.
Step-by-step solution
Step 1
Clean water supply: treating drinking water removes the pathogen, so it is not swallowed.
Step 2
Sewage treatment / proper waste disposal: stops faeces (containing the pathogen) contaminating water or food.
Step 3
Good hygiene: washing hands, especially after the toilet and before handling food, removes pathogens.
Step 4
Hygienic food preparation and storage: cooking food properly and keeping it covered prevents contamination.
Answer
Provide clean water, treat sewage/dispose of waste safely, practise good personal hygiene, and prepare and store food hygienically.
Examiner tip
For cholera, the most important measures are clean water and sewage treatment, because it spreads through contaminated water.
Question
Explain how the cholera bacterium causes the severe watery diarrhoea seen in cholera.
Step-by-step solution
Step 1
The cholera bacterium, taken in through contaminated water, multiplies in the small intestine and produces a toxin.
Step 2
The toxin causes the cells lining the intestine to secrete chloride ions into the gut.
Step 3
This lowers the water potential of the gut contents.
Step 4
So water moves out of the cells lining the intestine (and from the blood) into the gut by osmosis, causing watery diarrhoea, dehydration and loss of salts.
Answer
The cholera toxin makes the intestine cells secrete chloride ions, lowering the water potential of the gut contents, so water moves into the gut by osmosis — causing watery diarrhoea and dehydration.
Examiner tip
The chain is: toxin → chloride ions secreted → lower water potential in gut → water leaves by osmosis → diarrhoea. The osmosis step is essential.
Question
In a town, the number of cases of cholera fell sharply after a clean water supply and a sewage treatment system were introduced. Explain why these measures were so effective.
Step-by-step solution
Step 1
Cholera spreads when the bacterium in the faeces of infected people contaminates drinking water.
Step 2
Sewage treatment removes or kills the bacteria in human waste, so they cannot reach the water supply.
Step 3
A clean (treated) water supply means people no longer swallow water containing the bacterium.
Step 4
Breaking this transmission route — from faeces to water to mouth — stops new infections, so the number of cases falls sharply.
Answer
Cholera spreads via faeces contaminating water. Treating sewage and supplying clean water break this faeces-to-water-to-mouth route, so far fewer people swallow the bacterium and cases fall.
Examiner tip
Identify the specific transmission route and show how each measure interrupts it — that reasoning earns the marks.
High-scoring sample answers for diseases on the Cambridge IGCSE 0610 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.
State what is meant by a pathogen. (1 mark)
Model answer
A pathogen is a disease-causing organism.
Why this scores
One mark for 'disease-causing organism / microorganism'.
State two ways in which a transmissible disease can be spread from one person to another. (2 marks)
Model answer
Any two of: through contaminated food or water; through the air in droplets from coughs and sneezes; through direct contact with blood or body fluids; or carried by an animal (vector).
Why this scores
Two genuinely different routes. Direct (body fluids) and indirect (water/air/vector) both count.
Describe three ways the body defends itself against pathogens entering or surviving. (3 marks)
Model answer
The skin acts as a physical barrier that keeps pathogens out. Mucus in the airways traps pathogens, and cilia sweep the mucus up to the throat to be swallowed. Stomach acid (hydrochloric acid) kills most pathogens that are swallowed in food and drink.
Why this scores
Three marks, each a defence with how it works: skin (barrier); mucus traps / cilia sweep; stomach acid kills.
Describe how the spread of cholera in a community can be reduced. (4 marks)
Model answer
Providing a clean, treated drinking water supply ensures people do not swallow the cholera bacterium. Treating sewage and disposing of human waste safely prevents faeces, which contain the bacterium, from contaminating the water or food. Good personal hygiene, such as washing hands after using the toilet and before handling food, removes the bacterium. Preparing and storing food hygienically also prevents it being contaminated.
Why this scores
Four marks from: clean/treated water; sewage treatment/safe waste disposal; hand washing/personal hygiene; hygienic food preparation.
Explain how the cholera bacterium causes severe diarrhoea. (5 marks)
Model answer
After being taken in, usually through contaminated water, the cholera bacterium multiplies in the small intestine and produces a toxin. This toxin causes the cells lining the intestine to secrete chloride ions into the gut. The chloride ions lower the water potential of the contents of the gut. As a result, water moves out of the cells lining the intestine, and from the blood, into the gut by osmosis. This large amount of water in the gut causes the watery diarrhoea of cholera, which leads to dehydration and a loss of ions from the body.
Why this scores
Five marks: bacterium produces a toxin; toxin makes cells secrete chloride ions; this lowers the water potential of the gut contents; water moves into the gut by osmosis; causing diarrhoea/dehydration.
Explain how the body's natural defences and good hygiene together reduce the chance of becoming ill from pathogens. (6 marks)
Model answer
The body has several natural defences that act as the first line of protection. The skin forms a physical barrier that keeps pathogens out of the body. In the airways, mucus traps pathogens that are breathed in, and cilia on the lining cells sweep this mucus up to the throat to be swallowed, where the hydrochloric acid in the stomach kills most of the pathogens. These defences reduce the number of pathogens that get inside the body. Good hygiene reduces the chance of pathogens reaching the body in the first place: washing hands removes pathogens before they enter through the mouth; providing clean water and treating sewage prevents pathogens in faeces from contaminating drinking water; and preparing food hygienically stops it being contaminated. Together, the natural defences keep pathogens out of the body while good hygiene reduces the number of pathogens a person is exposed to, so the chance of infection is much lower.
Why this scores
Up to 6 marks: natural defences (skin barrier; mucus traps; cilia sweep; stomach acid kills) and hygiene measures (hand washing; clean water/sewage treatment; hygienic food) — with the idea that one keeps pathogens out and the other reduces exposure.
Definitions to memorise and the exact keywords mark schemes credit for diseases answers — sharpened from recent examiner reports for the 2026 0610 sitting.
A disease-causing organism. Includes bacteria, viruses, fungi and protoctists.
A disease in which the pathogen can be passed from one host to another (e.g. cholera, influenza).
An animal that carries a pathogen from one host to another. Example: the mosquito carries the malaria pathogen.
Tiny hair-like structures on cells lining the airways. They sweep mucus (with trapped pathogens) up to the throat.
Sticky fluid produced in the airways and gut. Traps pathogens and dust.
The traps other students keep falling into on diseases questions — taken from recent Cambridge IGCSE 0610 examiner reports and mark schemes — and how to avoid them.
Why it happens
Most pathogens are cellular.
How to avoid it
Viruses are NOT cells — they are a protein coat around genetic material, and can only reproduce inside a host cell.
Why it happens
It kills most.
How to avoid it
Say 'most' pathogens. Some survive and can cause disease.
Why it happens
The mosquito 'gives' you malaria.
How to avoid it
The vector CARRIES the pathogen but is not itself the cause. Malaria is caused by a protoctist; the mosquito is only the carrier.
The things students keep getting wrong in this sub-topic, answered.