IGCSE · Practice questions · Pathogens and body defences

Keep the pathogen out.

Six original Cambridge-style questions on pathogens and transmission, body defences, controlling the spread of disease, outbreak data and (Extended) the cholera toxin.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Organism, route, barrier.

Pathogen: a disease-causing organism. Transmissible: it can pass from one host to another.
Direct = contact, blood, body fluids. Indirect = surfaces, food, animals, air.
Defences: skin, nose hairs, mucus, stomach acid, white blood cells; say what each one does.
Cholera (Extended): toxin → chloride ions into gut → water follows by osmosis → diarrhoea, dehydration, loss of ions.

Recall

[3 marks]

Define pathogen and transmissible disease, and give one example of a disease that is not transmissible.

A pathogen is a disease-causing organism. ✓
A transmissible disease is a disease in which the pathogen can be passed from one host to another. ✓
Any non-transmissible disease, for example scurvy, rickets, coronary heart disease or lung cancer. ✓

Explain

[4 marks]

Describe how four of the body's defences prevent pathogens from causing disease.

Skin: a tough barrier that pathogens cannot cross unless it is broken; cuts are sealed by blood clotting. ✓
Hairs in the nose: trap dust and pathogens in the air breathed in. ✓
Mucus: traps pathogens in the airways; it is swept to the throat and swallowed. ✓
Stomach acid: hydrochloric acid kills pathogens swallowed in food and drink. (Also accept white blood cells: phagocytes engulf and digest pathogens, lymphocytes make antibodies.) ✓

Explain

[4 marks]

A city builds a new sewage treatment works and a piped supply of clean water. Explain how each of these reduces the number of people who catch transmissible diseases.

Many pathogens, such as the cholera bacterium, are passed out in human faeces. ✓
Sewage treatment removes these pathogens before the water returns to rivers, lakes and wells. ✓
A clean water supply means drinking water does not contain pathogens. ✓
So fewer people take in pathogens indirectly through contaminated water, and the chain of transmission is broken. ✓

Data

[4 marks]

A school recorded the number of students absent with a stomach infection each week. Hand-washing stations were installed at the start of week 4.

Week	1	2	3	4	5	6
Students absent	12	18	24	15	8	4

(a) Calculate the percentage decrease from week 3 to week 6. (b) Suggest an explanation for the change and one reason why the data do not prove that hand washing caused it.

(a) (24 − 4) ÷ 24 × 100 = 83% (83.3%) decrease. ✓
(b) Washing hands removes pathogens picked up from surfaces and the toilet, so fewer are carried to the mouth or onto food. ✓
So fewer students were infected indirectly and the number absent fell. ✓
It does not prove cause: other factors may have changed (for example the outbreak may have run its course as students became immune, or the canteen improved food hygiene); there was no control group. ✓

Explain

[4 marks]

Extended. Explain how the cholera bacterium causes diarrhoea and dehydration.

The bacterium produces a toxin in the small intestine. ✓
The toxin causes the cells lining the intestine to secrete chloride ions into the gut. ✓
This lowers the water potential of the gut contents, so water moves out of the cells and blood into the gut by osmosis. ✓
The large volume of watery faeces is diarrhoea; the body loses water and ions from the blood, causing dehydration. ✓

Analysis

[3 marks]

Extended. After a flood, many people in a village catch cholera. Explain why the flood led to the outbreak and suggest why patients are given a drink of water containing salts and glucose.

Flood water mixes sewage containing cholera bacteria with drinking water sources such as wells. ✓
Cholera is transmitted in contaminated water, so people who drink it are infected. ✓
The drink (oral rehydration solution) replaces the water and ions lost from the blood in diarrhoea, preventing dangerous dehydration. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.