Pathogens and transmission — Cambridge IGCSE Biology
What pathogens are, how they spread, the body defences, hygiene and sanitation, and cholera.
Revision notes
4 short notes, in the order of the specification. Each one in short:
A pathogen is a disease-causing organism, and bacteria, viruses, fungi and protists can all be pathogens. A transmissible disease is one in which the pathogen can be passed from one host to another. Transmission can be direct, through blood and other body fluids, or indirect, through contaminated surfaces, food, water, animals or air.
The body's defences against pathogens include barriers that keep them out and cells that destroy those that get in. The skin is a tough barrier, nose hairs and mucus trap pathogens, and stomach acid kills most swallowed pathogens. White blood cells destroy pathogens inside the body by ingesting them or by making antibodies.
The spread of disease is controlled by breaking transmission routes so fewer people take in pathogens. The main measures are a clean water supply, hygienic food preparation, good personal hygiene, waste disposal and sewage treatment. For example, treating drinking water and sewage stops water-borne diseases such as cholera from spreading.
Cholera
SupplementCholera is a disease caused by a bacterium transmitted in contaminated water. The bacterium produces a toxin in the small intestine that makes the cells lining it secrete chloride ions. This lowers the water potential in the gut, so water moves in from the blood by osmosis, causing diarrhoea, dehydration and loss of ions from the blood.
Exam questions
8 exam-style questions (21 marks), each with its mark scheme.
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12 cards: flip them, mark what you knew, and practise the rest.
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The whole of diseases and immunity on one page, so you can see where this subtopic fits.
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Quick check questions
What is a pathogen?
Show answer
A disease-causing organism.
What is a transmissible disease?
Show answer
A disease in which the pathogen can be passed from one host to another.
Name the body defences in the syllabus.
Show answer
Skin, hairs in the nose, mucus, stomach acid and white blood cells.
Supplement Which route of transmission is cholera associated with?
Show answer
Contaminated water.
Frequently asked questions
How do pathogens spread?
Pathogens spread by direct contact or indirectly. Direct contact includes blood and other body fluids passing from one person to another, for example through a cut. Indirect routes are contaminated surfaces such as door handles, contaminated food and water, animals such as mosquitoes, and tiny droplets in the air from coughs and sneezes.
How does the body stop pathogens getting in?
The skin forms a tough barrier that stops pathogens entering the body. Hairs in the nose and sticky mucus in the airways trap pathogens so they can be removed, and stomach acid kills most pathogens swallowed in food and drink. White blood cells then destroy any pathogens that do get inside the body.
Why is clean water important for preventing disease?
Clean water is important because drinking water can carry pathogens. Treating the water supply removes them and stops water-borne diseases such as cholera from spreading. Sewage treatment also matters, because sewage contains pathogens from human faeces, and treating it stops them contaminating drinking water or food crops.
How does cholera cause diarrhoea?
Supplement The cholera bacterium produces a toxin in the small intestine. The toxin makes the cells lining the small intestine secrete chloride ions, which lower the water potential in the gut. Water then moves from the blood into the gut by osmosis, and the extra water causes diarrhoea, so the person becomes dehydrated and loses ions.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026