The shape of the lesson
By the end of the lesson, learners can
- Pathogens and their routes: Press Start an outbreak.
- The body's defences: Pick each defence in turn.
- Cholera: water leaves the blood: Extended.
- Breaking the chain: Back in the outbreak, switch on the controls one at a time and run the outbreak again.
- 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.
One picture every idea returns to
A pathogen is a disease-causing organism. A transmissible disease is one in which the pathogen can be passed from one host to another, either by direct contact (including blood and other body fluids) or indirectly (from contaminated surfaces, food, animals and air). The body keeps most pathogens out with skin, nose hairs, mucus and stomach acid, and white blood cells deal with those that get in. Clean water, hygienic food preparation, personal hygiene, waste disposal and sewage treatment break the routes of transmission.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, How disease spreads, and how we stop it.. Some diseases jump from person to person; others never do. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 11 min | Teach and check: Pathogens and their routes | Press Start an outbreak. Project tab 1 of the lab (An outbreak spreading through a town) and let a student drive it while the class predicts. | Whole class, then pairs |
| 11 to 17 min | Teach and check: The body's defences | Pick each defence in turn. Project tab 2 of the lab (Defences of the human body) and let a student drive it while the class predicts. | Pairs |
| 17 to 23 min | Teach and check: Cholera: water leaves the blood | Extended. Project tab 6 of the lab (How the cholera toxin causes dehydration) and let a student drive it while the class predicts. | Pairs |
| 23 to 30 min | Teach and check: Breaking the chain | Back in the outbreak, switch on the controls one at a time and run the outbreak again. | Jigsaw |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 22 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Breaking the chain, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Jigsaw
Split the content into expert pieces. Expert groups master one piece each, then regroup so every home team has one expert per piece, and each expert teaches their piece to the team.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| A pathogen is the organism that causes the disease, not the disease itself. | Cholera is the disease; the cholera bacterium is the pathogen. |
| Stomach acid kills pathogens; it does not "digest" them like food. | And skin is a barrier only while it is unbroken, which is why a cut must clot quickly. |
| The toxin does not pump water directly. | It moves chloride ions into the gut, and water follows by osmosis. Mention both steps in an explanation. |
| Hygiene measures stop pathogens reaching people. | They do not make anyone immune, so they must be kept up all the time. |
Pathogens and body defences: worksheet
6 questions, 22 marks. Written in the style of the papers; no past paper question is reproduced.
Define pathogen and transmissible disease, and give one example of a disease that is not transmissible.
Describe how four of the body's defences prevent pathogens from causing disease.
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.
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.
Extended. Explain how the cholera bacterium causes diarrhoea and dehydration.
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.
Answer key and marking notes
- 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.
- 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.)
- 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.
- (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.
- 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.
- 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.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which of these is a pathogen?
- AThe bacterium that causes cholera
- BThe toxin released by the cholera bacterium
- CAn antibody made by a lymphocyte
- DThe diarrhoea caused by an infection
Which is an example of indirect transmission of a pathogen?
- ACatching food poisoning from a chopping board used for raw chicken
- BCatching HIV from infected blood given in a transfusion
- CCatching a skin infection by touching an infected person's skin
- DDeveloping scurvy from a diet that lacks vitamin C
How do the hairs in the nose help to protect the body against disease?
- AThey trap dust and pathogens in the air breathed in
- BThey kill pathogens with hydrochloric acid
- CThey engulf and digest pathogens in the nose
- DThey sweep mucus up from the lungs to the throat
Houseflies breed in open rubbish heaps and then land on food. Which measure most directly stops flies spreading pathogens in this way?
- ACollecting rubbish and disposing of it safely
- BAdding chlorine to the drinking water supply
- CTreating sewage before it reaches the river
- DVaccinating everyone against the disease
The table shows the number of cases of a water-borne disease in a town over four years. A water treatment plant opened at the start of year 3. What is the percentage decrease in cases from year 2 to year 4?
- A87%
- B13%
- C75%
- D650%
Extended: which sequence correctly describes how cholera causes dehydration?
- AToxin → chloride ions secreted into the gut → water follows by osmosis → diarrhoea
- BToxin → water secreted into the gut → chloride ions follow by diffusion → diarrhoea
- CToxin → chloride ions absorbed into the blood → water follows into the blood → diarrhoea
- DBacteria absorb water from the gut → blood becomes concentrated → diarrhoea
Extended: a child with cholera is given a drink containing water, glucose and salts rather than plain water. What is the main reason for including the salts?
- ATo replace the ions lost from the blood in diarrhoea
- BTo kill the cholera bacteria in the small intestine
- CTo neutralise the toxin made by the bacteria
- DTo act as a vaccine against a second infection
Some people take medicines that greatly reduce the acid made by the stomach. Suggest why these people are more likely to catch food poisoning.
- AFewer of the pathogens swallowed in food are killed in the stomach
- BThe skin becomes thinner and easier for pathogens to cross
- CLess mucus is made, so pathogens in food are not trapped
- DWhite blood cells need stomach acid to make antibodies
Quiz key
A pathogen is a disease-causing organism, so the cholera bacterium is the pathogen. The toxin is a chemical the bacterium makes, not an organism. An antibody is a protein made by the body to fight pathogens. Diarrhoea is a symptom of the disease, not the thing that causes it.
The chopping board is a contaminated surface that carries the pathogen to the food, so transmission is indirect. Infected blood in a transfusion and skin-to-skin touching are both direct contact, including through blood and body fluids. Scurvy is a deficiency disease with no pathogen, so it is not transmissible at all.
Nose hairs act as a filter, trapping dust and pathogens before they go further into the airways. Hydrochloric acid is made in the stomach, not the nose. Engulfing and digesting pathogens is done by phagocytes, a type of white blood cell. Mucus is moved up from the airways to the throat by cilia, which are tiny hair-like projections on cells, not the hairs in the nose.
Safe waste disposal removes the rubbish that attracts and feeds flies, so fewer flies carry pathogens to food. Chlorinating water and treating sewage break the route through contaminated water, not through flies. Vaccination makes people immune to one particular disease; it does not stop flies carrying pathogens, and there are no vaccines for many of the diseases flies spread.
The decrease is 450 − 60 = 390 cases, and 390 ÷ 450 × 100 = 86.7%, so about 87%. 13% is the year 4 value as a percentage of year 2 (the fraction remaining, not the decrease). 75% compares year 3 with year 4 instead of year 2 with year 4. 650% divides by the final value (390 ÷ 60) instead of the starting value.
The toxin makes the cells lining the small intestine secrete chloride ions into the gut. This lowers the water potential of the gut contents, so water moves by osmosis out of the cells and blood into the gut, causing diarrhoea and dehydration. The ions move first and water follows, not the other way round. If the ions moved into the blood, water would follow into the blood, not into the gut. The bacteria do not absorb water to cause the disease; their toxin does the damage.
Cholera causes loss of water and ions from the blood, so the salts in oral rehydration solution replace the lost ions while the water replaces the lost fluid. Salts do not kill bacteria; that would need an antibiotic. They do not neutralise the toxin. A vaccine contains weakened pathogens or antigens, not salts.
Hydrochloric acid in the stomach kills most of the pathogens swallowed in food and drink, so with less acid more survive to reach the intestine and cause disease. Stomach acid has no effect on the skin. Mucus in the airways traps pathogens breathed in, not pathogens in food. Lymphocytes make antibodies without any need for stomach acid.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.