Hi Tara. One piece of housekeeping before you start, so that the real syllabus document does not confuse you. Cambridge numbers the whole of Topic 10 as a single section, 10.1. There is no official 10.2 or 10.3. I have split it into five for teaching, because five short pieces are far easier to learn and to revise than one long one, and because the papers on this site report your score by sub-topic. The content is identical to the syllabus — only the numbering is mine.
Here is the shape of it. 10.1 is two definitions and two routes: what a pathogen is, what makes a disease transmissible, and the difference between direct and indirect transmission. 10.2 is a list of five body defences and five public health measures — the easiest marks in the topic, and the ones most often written too vaguely to score. 10.3 is the heart of the topic: antigens, antibodies, lymphocytes, memory cells and vaccination, and it is where the antigen/antibody swap destroys answers. 10.4 is passive immunity, which is defined almost entirely by what it does not have — memory cells. 10.5 is cholera, one disease examined in real mechanistic detail, and the only place in the syllabus where you will be asked to explain diarrhoea using the word osmosis. Then 10.6 is a checklist for the night before a paper.
Two Definitions, and They Are Worth Learning Word for Word
Everything else in this topic sits on top of these two sentences, and both of them are one-mark answers that people write in three vague lines instead.
A pathogen is a disease-causing organism.
A transmissible disease is one in which the pathogen can be passed from one host to another.
Look at what the first definition does not say. It does not say “a germ”, it does not say “bacteria”, and it does not say “something that makes you ill”. It says organism, which means a living thing, and that single word covers four very different groups: bacteria (cholera, tuberculosis), viruses (influenza, measles, HIV), fungi (athlete’s foot) and protoctists (malaria). You met all four kingdoms in Topic 1, and this is where that classification finally earns its keep — a question that asks “name the type of organism that causes cholera” is not asking for “a germ”, it is asking for bacterium.
Everyday English has three words for this and Cambridge uses exactly one. Write transmissible. And notice what the definition is really testing: it is testing whether you know that not every disease is transmissible. Scurvy, rickets and anaemia are diseases, and you cannot catch any of them from a person who has one, because there is no pathogen to pass on. A deficiency disease has a cause, not a culprit.
The word host also matters. A host is the organism the pathogen is living in and doing damage to. Transmission is the journey from one host to the next, and the whole of 10.1 is a description of the ways that journey can be made.
Direct and Indirect — One Distinction, and It Is Simpler Than It Looks
Cambridge splits transmission into two routes, and candidates lose marks here for a surprising reason: they try to make the distinction about distance, or about how nasty the route sounds. It is neither. The test is a single question:
Was there anything in between?
If the pathogen went straight from one host to the other with nothing in the middle, that is direct transmission. If it spent any time on or in something else — a surface, some food, an animal, the air — that is indirect transmission.
| Route | What the syllabus says | Worked examples |
|---|---|---|
| Direct | Including through blood and other body fluids | A contaminated needle; a blood transfusion; sexual contact; touching an infected skin lesion; a pathogen crossing the placenta from mother to fetus |
| Indirect | From contaminated surfaces | Door handles, taps, cups, phones — anything a person touches after an infected person has touched it |
| From contaminated food | Food prepared with unwashed hands; drinking water containing sewage — this is how cholera travels | |
| From animals | A mosquito carrying the malarial parasite; a housefly walking from faeces onto food. An animal that carries a pathogen from one host to another is called a vector. Whether the animal itself is harmed does not matter | |
| From the air | Droplets of mucus and saliva launched by a cough or a sneeze and breathed in by someone else — influenza, tuberculosis, the common cold |
Why This Sub-Topic Is Really About the Rest of the Paper
It is tempting to treat 10.1 as a page of vocabulary and move on. Do not, because the transmission route decides the answer to almost every applied question in the topic. If you know cholera travels in contaminated water, then you already know that the way to stop an outbreak is a clean water supply and sewage treatment, and you already know that vaccinating a village whose well is contaminated will help far less than fixing the well. If you know influenza travels in the air, you know that hand-washing helps but does not solve it.
So whenever a question hands you an unfamiliar disease, your first move is always the same: find the route. The route tells you the control measure, and the control measure is usually where the marks are.
The stem tells you it is a bacterium, so that half is free — but you must write the word, not “a germ”. It is transmissible, because the pathogen is passed from one host to another: from a goat to a person. Note that the definition does not require both hosts to be the same species.
Milk: was anything in between? Yes — the milk. That is indirect transmission through contaminated food. Cuts during birth: the pathogen passes straight from the goat’s body fluids into the worker’s blood with nothing in between, so that is direct.
Two routes means two different control measures, and a follow-up question will usually ask for them. Boiling or pasteurising the milk closes the indirect route; gloves close the direct one. Neither closes both, which is exactly the point such a question is testing.
A cough sends droplets straight from one person’s lungs into another person’s face, so it feels direct. It is not. The droplets travel through the air, and the air is the thing in between — the syllabus lists the air explicitly under indirect transmission. This is the single most commonly missed classification in the topic, and it is worth memorising as a flat fact rather than reasoning about each time.
Five Defences, and the Word “Limited” in the Syllabus
The syllabus does something unusual here. It says the body defences are limited to five things — skin, hairs in the nose, mucus, stomach acid, white blood cells — which means that is the whole examinable list. That is good news and it has a consequence: if a question asks for three defences, all three must come from those five. Tears, sweat, earwax, coughing and sneezing are all real defences and none of them will be on the mark scheme.
Physical barriers — they stop the pathogen getting in.
• Skin: a continuous, tough, dry outer layer that pathogens cannot pass through while it is unbroken.
• Hairs in the nose: they trap dust and larger particles in the air before it goes any further.
• Mucus: a sticky liquid lining the nose, trachea and bronchi. Pathogens stick to it and are swept away.
A chemical barrier — it destroys the pathogen.
• Stomach acid: hydrochloric acid at about pH 2 kills most pathogens swallowed in food and drink.
A cellular defence — it deals with whatever gets past the first four.
• White blood cells: phagocytes carry out phagocytosis; lymphocytes produce antibodies.
Grouping them like this is not decoration. It is what turns a list into an explanation, and “explain how” questions are marked on the mechanism, not the name. “Mucus” scores one mark at most; “mucus lines the trachea and traps pathogens, which are then swept away by the ciliated cells” scores whatever the question is worth. You met those ciliated cells in Topic 2 as an example of a specialised cell — this is the job they were specialised for.
The Two White Blood Cells — Keep Them Apart From the Start
You met both of these in Topic 9. They are worth restating here because 10.3 is about to be built entirely on one of them.
| Cell | What it does | The word Cambridge wants |
|---|---|---|
| Phagocyte | Engulfs the pathogen, takes it inside the cell and digests it. It does this to any pathogen — it does not need to have met it before | Phagocytosis. Not “eats”, not “absorbs”, not “kills” |
| Lymphocyte | Produces antibodies, and each kind of antibody works against only one kind of pathogen | Antibody production. The lymphocyte makes the antibody; it does not swallow anything |
Phagocyte – swallow. Lymphocyte – label. One takes the pathogen inside itself; the other makes a molecule that sticks to the outside of the pathogen. If your answer has a lymphocyte engulfing something or a phagocyte producing an antibody, the two cells have swapped jobs and the mark is gone.
Controlling the Spread — Five Measures, and Why Each One Works
The syllabus asks you to explain the importance of five public health measures. “Explain” means a because. The name of the measure is never the whole mark, and this is where candidates who have learned the list still score badly.
| Measure | Which transmission route it closes | The explanation that scores |
|---|---|---|
| A clean water supply | Contaminated water | Water that has been treated contains no pathogens, so people do not swallow them when they drink, cook or wash food. This is the single most important measure against cholera |
| Hygienic food preparation | Contaminated food | Washing hands and surfaces, keeping raw and cooked food apart, and cooking food thoroughly all destroy or remove pathogens before the food is eaten |
| Good personal hygiene | Contaminated surfaces and hands; some direct contact | Washing hands, especially after using the toilet and before handling food, removes pathogens from the hands so they are not transferred to the mouth or to other people |
| Waste disposal | Animals and contaminated surfaces | Rubbish left uncollected attracts flies and rats, which act as vectors; removing it removes the breeding sites and so removes the carriers |
| Sewage treatment | Contaminated water and food | Untreated sewage carries pathogens from faeces. Treating it stops those pathogens reaching drinking water, rivers and crops — it closes the loop that hygiene alone cannot |
They feel like the same idea and the syllabus lists them separately, so a question asking for two measures will accept both. Keep the logic distinct: sewage treatment deals with what leaves people, clean water supply deals with what reaches people. A town can do one and not the other, and in a real outbreak that is usually exactly what has happened. The syllabus adds “details not required” after sewage treatment, which means you are never asked how a treatment works — only why it matters.
The change is 210 − 12 = 198. The original is 210, not 12 and not 84. So 198 ÷ 210 × 100 = 94.3 %. Writing the working earns the method mark even if the arithmetic slips.
Clean piped water closes one route: drinking contaminated water. It does nothing about sewage leaking into rivers where people wash, into fields where crops are grown, or onto the feet of flies. Those are separate indirect routes, through contaminated food and through animals.
Sewage treatment removes pathogens from human waste before it is released, so it stops them reaching food crops, rivers used for washing, and the flies that visit both. That is why the two measures are listed separately in the syllabus — each closes a different door.
The Definition, and the Two Words That Have to Stop Being Confusable
Active immunity is defence against a pathogen by antibody production in the body.
The five words that matter are antibody production in the body. It is your lymphocytes doing your work. That is what “active” means — not that it acts quickly, but that the body is the one doing the acting.
Now the pair of words that decides whether you can do this topic at all.
| Antigen | Antibody | |
|---|---|---|
| What is it? | A molecule on the surface of the pathogen | A protein made by the body |
| Where does it come from? | The pathogen brings it in with it | Produced by a lymphocyte |
| What does it do? | Nothing helpful to you. It is simply a shape the body can recognise as foreign | Binds to the antigen, leading to direct destruction of the pathogen or to the pathogen being marked for destruction by phagocytes |
| How specific? | Each pathogen has its own antigens, and they have specific shapes | A specific antibody has a shape complementary to one antigen, and fits no other |
“A vaccine contains antibodies.” It does not. A vaccine contains antigens — weakened pathogens, or just their antigens. If the vaccine already contained antibodies there would be no reason to wait weeks for protection, no reason for memory cells to exist and no reason why a vaccine works for years while an injection of antibodies works for weeks. One swapped word makes the whole of 10.3 and 10.4 impossible to explain, and examiners know it, which is why it is asked in some form on almost every paper.
A sentence worth memorising: antigens are on the pathogen, antibodies are made by the lymphocyte. If you can say which structure each one is attached to, you cannot get it backwards.
The word complementary also needs care. Cambridge does not accept “the same shape”. An antibody does not look like an antigen any more than a key looks like a lock — it is the shape that fits into it. You have used this word before, in Topic 5, about an enzyme’s active site and its substrate. It is the same idea, and it is worth noticing that it is the same idea, because it means the specificity argument you already know transfers here for free.
What an Antibody Actually Does Once It Has Bound
Cambridge gives two outcomes and a good answer names them separately, because “the antibody destroys the pathogen” is only half the story and is often the half that is not being asked about.
An antibody binds to the antigen, and this leads either to
1. Direct destruction of the pathogen — the antibody itself finishes the job, for example by causing pathogens to stick together in clumps or by damaging them.
2. Marking of the pathogen for destruction by phagocytes — the antibody acts as a label, and a phagocyte then engulfs and digests the labelled pathogen.
The second outcome is the one candidates leave out, and it is the one that ties this sub-topic back to the phagocytes of 10.2. The two white blood cells are not rivals; they work together, with the lymphocyte pointing and the phagocyte doing the swallowing.
Vaccination — Three Numbered Steps, and Cambridge Wants All Three
The syllabus sets this out as a numbered process, which is a very strong hint about how it is marked. Learn it as three steps with a fourth consequence.
Why the Second Exposure Is So Different From the First
This graph appears on Paper 4 constantly, usually with no labels and a request to describe and explain the difference between the two peaks. There are three differences and each is a mark: the response is faster, larger and lasts longer.
The chain is worth writing out in full because it is a four-mark answer waiting to happen: memory cells recognise the antigen → antibodies are produced faster and in greater quantity → the pathogen is destroyed before it can multiply enough → no symptoms develop. The person is still infected in the sense that the pathogen entered; they simply never become ill.
Two Ways to Become Actively Immune
Active immunity is gained either after an infection by a pathogen or by vaccination. Both work by exactly the same mechanism and produce exactly the same memory cells. The difference is only in the price you pay to get them.
| By infection | By vaccination | |
|---|---|---|
| What enters the body | The live pathogen, multiplying | A weakened pathogen or just its antigens |
| Antibodies produced by | Your own lymphocytes | Your own lymphocytes |
| Memory cells produced? | Yes | Yes |
| Do you become ill? | Yes, and you may not survive it | No, because the pathogen is weakened and cannot cause the disease |
“Suggest why vaccination is preferable to gaining immunity by catching the disease.” The mark is not for saying vaccination is safer. It is for the mechanism: the vaccine contains a weakened pathogen or its antigens, so it stimulates the same immune response and the same memory cells without causing the disease. Say that and you have both marks.
The Role of Vaccination in Controlling Spread
Vaccination protects the individual, and that part is obvious. The syllabus asks for something one step further: how vaccination controls the spread of a disease through a population. The reasoning is short and it is worth having ready.
1. A vaccinated person does not develop the disease, so the pathogen does not multiply in them.
2. Because it does not multiply in them, they do not pass it on to anyone else.
3. If a large enough proportion of the population is vaccinated, the pathogen runs out of people to move to and transmission stops.
4. This protects the people who cannot be vaccinated — newborn babies, people who are already very ill — because they are surrounded by people who cannot pass the disease on. If enough people are vaccinated for long enough, the disease can be wiped out entirely, which is what happened to smallpox.
The first dose produced antibodies and memory cells. When the booster arrives, the memory cells recognise the antigen immediately, so there is almost no lag, and antibodies are produced much faster and in much greater quantity — 46 units in 5 days against 4 units in 21 days. Quoting both pairs of figures is very often a mark of its own.
A different pathogen has different antigens, with different shapes. The antibodies the child already has are complementary only to the first antigen and cannot bind to the new one, so they give no protection. Being immune to one disease says nothing about any other, however similar the two organisms are.
Nothing happens inside the baby at all. The vaccinated children do not develop the disease, so they do not pass the pathogen on; with 90 % of the population unable to transmit it, the chance of the pathogen reaching the baby becomes very small. Answers that say the baby “gets some immunity from the others” are describing something that does not happen.
One Definition, Three Clauses, and Every Clause Is a Mark
Passive immunity is a short-term defence against a pathogen by antibodies acquired from another individual.
Three ideas: it is short-term; it works by antibodies; and those antibodies were made by somebody else.
Read that against the definition of active immunity and you will see they are built from exactly the same words, arranged to mean opposite things. Active immunity is antibody production in the body. Passive immunity is antibodies acquired from another individual. In one case you make them; in the other you are handed them ready-made.
The syllabus names two natural routes, and both involve a mother and her baby:
- Across the placenta, before birth. The mother’s antibodies cross into the fetus’s blood, so a newborn arrives already carrying protection against the diseases its mother is immune to.
- In breast milk, after birth. Antibodies in the milk pass to the infant and continue to protect it while it feeds.
A newborn baby has an immune system that is not yet fully developed and has had almost no chance to meet pathogens or to be vaccinated, so it produces very few antibodies of its own. Breast milk supplies ready-made antibodies from the mother, giving immediate protection during exactly the months when the baby is least able to protect itself. The protection is temporary and fades as the baby stops feeding — which is precisely why the childhood vaccination programme begins at a few weeks old, before that borrowed protection has run out.
The Missing Piece: No Memory Cells
This is the sentence that separates the two kinds of immunity, and Cambridge states it explicitly: memory cells are not produced in passive immunity.
The reason follows from the definition, so you never have to memorise it as a separate fact. Memory cells are made by lymphocytes that have been stimulated by an antigen. In passive immunity no antigen ever enters the body — only finished antibodies do. The lymphocytes are never stimulated, so they make nothing, and when the borrowed antibodies are broken down there is nothing left behind. Protection ends completely.
| Active immunity | Passive immunity | |
|---|---|---|
| Who made the antibodies? | The person’s own lymphocytes | Another individual |
| What triggers it? | An antigen — from an infection or a vaccination | Nothing. Finished antibodies simply arrive |
| How quickly does protection appear? | Slowly — days to weeks | Immediately |
| How long does it last? | Long-term, often for life | Short-term — weeks to months |
| Memory cells? | Yes | No |
| Examples | Recovering from measles; being vaccinated | Antibodies crossing the placenta; antibodies in breast milk |
Trap one: “passive means slow”. It is the exact opposite. Passive immunity is the fast one, because the antibodies are already made. Active is the slow one, because your lymphocytes have to be stimulated first and then have to make everything from scratch. The words describe who does the work, not how long it takes.
Trap two: “breast milk protects the child for life”. No memory cells are produced, so once the antibodies are broken down the protection is completely gone. A child who was breastfed still needs every vaccination on the schedule.
Passive immunity, because the antibodies were acquired from another individual — the mother. They crossed the placenta before birth, and more may have arrived in breast milk afterwards. Two marks, two separate ideas: the name and the route.
Antibodies are protein molecules and are gradually broken down. The baby cannot replace them, because no memory cells were produced — no antigen ever entered the baby, so its lymphocytes were never stimulated. Nothing is producing more, so the concentration falls to zero.
The vaccine contains weakened pathogen or its antigens; the antigens stimulate the baby’s own lymphocytes, which produce antibodies and, crucially, memory cells. The memory cells persist, so if the real measles virus arrives years later, antibodies are produced faster and in greater quantity and the child does not become ill. That is active immunity, and it is long-term precisely because of the step passive immunity lacks.
The Three Facts, Then the Mechanism
Cholera is the only disease in the whole of Topic 10 that you have to know in mechanistic detail, and that is a strong hint: if a Paper 4 asks you to explain a disease, it will be this one. Start with the three flat facts.
Caused by a bacterium.
Transmitted in contaminated water.
The symptoms are caused by a toxin, not by the bacterium damaging the gut.
That third line is the one people miss, and it changes every answer that follows. The cholera bacterium does not burrow into the intestine wall, does not eat it and does not kill the cells. It sits in the small intestine, multiplies, and releases a toxin — a poisonous chemical — and it is the toxin that does everything. This matters because it explains why the illness begins so suddenly and why the gut recovers completely once the bacteria are gone.
The Mechanism — Four Steps, and Step Three Is Osmosis
Here is the chain the syllabus asks for. Learn it as four linked steps, because a four-mark question is asking for exactly these four and no more.
- The cholera bacterium in the small intestine produces a toxin.
- The toxin causes the cells lining the small intestine to secrete chloride ions into the small intestine.
- This lowers the water potential of the contents of the gut, so water moves into the gut by osmosis, from the cells and blood, through partially permeable membranes.
- The result is diarrhoea, and therefore dehydration and loss of ions from the blood.
Writing that the toxin “draws water into the gut” or “pumps water out of the blood”. Nothing moves the water. The toxin moves chloride ions; that makes the gut contents more concentrated, which means a lower water potential; and water then moves down its own gradient by osmosis, entirely passively. Miss out the ions and you have skipped the only step that explains anything. Miss out the word osmosis and you have skipped the only step the mark scheme names.
Two consequences, and they are separate marks. Dehydration: so much water has been lost into the gut and then out of the body that the blood volume falls. Loss of ions from the blood: the chloride and other ions leave with the water, and the concentration of the blood plasma is disturbed. That is why treatment is a drink containing water and ions, not water alone — replacing water without replacing the ions leaves the plasma too dilute. A stem may hand you this treatment and ask you to explain it; the explanation is the mechanism read backwards.
Prevention Follows From the Transmission Route
Cholera is transmitted in contaminated water, so everything in 10.2 that touches water is the answer: a clean water supply, and sewage treatment so that faeces from an infected person never reach the drinking water in the first place. Hygienic food preparation and personal hygiene matter too, because food washed in contaminated water and hands not washed after using the toilet both carry the bacterium onward.
This is also a good place to notice a limit. Vaccination is a magnificent answer to measles and a partial answer to cholera. If the well is contaminated, everyone drinking from it is being exposed repeatedly and in large doses, and closing the route does far more than raising the defences. When a question asks for the most effective measure, read the transmission route in the stem before you answer.
The bacterium produces a toxin, and the toxin causes the cells lining the small intestine to secrete chloride ions into the gut. That is where the answer must begin, because it is the only step that has a cause outside Topic 3.
A high concentration of ions in the gut contents means the contents have a lower water potential than the cells and blood. Water therefore moves by osmosis from the higher water potential (the blood and cells) to the lower water potential (the gut), through the partially permeable cell membranes.
So much water enters the gut that the contents cannot be reabsorbed by the colon at anything like that rate, and the faeces are watery. Four marks: toxin; chloride ions secreted; lower water potential in the gut; water in by osmosis.
The patient has lost ions as well as water. Pure water would replace only half of what was lost and would dilute the blood plasma further; the drink must contain ions too. (The glucose is there because absorbing it helps the water to be absorbed with it, and because it supplies energy.)
The route is contaminated water. While the water supply remains contaminated everyone is exposed repeatedly, including people the vaccination programme misses; treating the water and the sewage closes the route for the entire village at once.
The Six Sentences That Earn Nothing
Each of these is written by thousands of candidates every year, sounds entirely reasonable, and is refused. Eliminating them is worth more than learning any new content.
| Never write | Write instead | Why |
|---|---|---|
| “The vaccine contains antibodies” | “The vaccine contains weakened pathogens or their antigens” | Antibodies are what your body then makes. If they were in the vaccine there would be no memory cells and no long-term protection |
| “The antibody has the same shape as the antigen” | “The antibody has a shape complementary to the antigen” | A key is not the same shape as a lock. Cambridge marks the word complementary |
| “White blood cells eat the antigens” | “Phagocytes engulf and digest the pathogen by phagocytosis” | Two errors at once: eat is not a marking word, and an antigen is a molecule on the pathogen, not the thing engulfed |
| “Passive immunity is slower because the body has to make antibodies” | “Passive immunity is immediate because the antibodies are already made” | The word passive describes who did the work, not the speed. Passive is the fast one |
| “The cholera toxin draws water into the gut” | “The toxin causes chloride ions to be secreted, lowering the water potential so water enters by osmosis” | The mechanism has an ion step. Skip it and there is nothing to explain the water movement |
| “A germ got into the body” | “A pathogen entered the body” | Germ is not a biological term and appears on no mark scheme |
The Antigen / Antibody Check, and How to Use It Under Pressure
Before you write either word, ask: where is it, and who made it?
Antigen — on the pathogen, made by the pathogen. It came in from outside.
Antibody — in your blood, made by your lymphocyte. It was built inside you.
If you find yourself writing that something “produces antigens” about a human cell, or “a vaccine full of antibodies”, the check has just caught an error that would have cost several marks in different questions.
Reading the Command Word
State / Name — one word or one short phrase. “Name the type of organism that causes cholera” wants bacterium, and nothing else is needed.
Describe — say what happens. For a graph this means the shape and the figures: which is higher, how much higher, how much sooner.
Explain — say why. In Topic 10 the explaining words are almost always memory cells, complementary, antigen or osmosis. If none of them appears in an explain answer, look again.
Outline — give the steps in order. This is the command word used for vaccination, and the order is the mark: antigen, lymphocyte, antibody, memory cell.
Suggest — apply what you know to an unfamiliar disease. Find the transmission route in the stem first; it is nearly always the key.
Compare — make statements that mention both things. “Active immunity lasts longer” is a comparison; “active immunity lasts a long time” on its own is not.
How to Attack the Graph Question
Topic 10 has essentially two graphs — primary against secondary response, and active against passive immunity — and one of them appears on almost every Paper 4. Work through them in the same order every time.
1. Read the axes and the arrows. When was the antigen given? Is there one exposure or two? An unlabelled second arrow is the whole question.
2. Find the starting value. A curve that starts at a maximum means the antibodies were not made by this person — that is passive immunity, every time.
3. Describe with figures. Three times higher, seven days sooner. Quoting the data is very often a mark on its own.
4. Explain with cells. A description mentions the curve; an explanation mentions memory cells and lymphocytes.
5. Check the direction of your claim. Faster means the rise is steeper and earlier, not that the peak is higher. Those are two separate differences and two separate marks.
Three Scenarios to Test Yourself On
Vaccinated children produced antibodies and memory cells, so they did not develop the disease and therefore could not pass the pathogen on. With almost everyone unable to transmit it, the pathogen ran out of new hosts and transmission stopped.
At 70 % there are enough unvaccinated people for the pathogen to move between, so the chain of transmission is re-established. Babies are hit hardest because they are too young to have been vaccinated and their passive immunity from their mothers has faded — and no memory cells were left behind by it.
This question is testing whether you can move from one individual to a population. The individual mechanism is memory cells; the population mechanism is transmission. Both are needed for full marks.
The vaccine does not contain antibodies. It contains weakened pathogens or their antigens. This is the error that makes the rest of the sentence impossible.
Nothing is killed at the moment of vaccination, because the weakened pathogen was never able to cause disease. What actually happens is that the antigens stimulate lymphocytes.
The body does not “remember antibodies”. It produces memory cells, which are cells, and they remember the antigen. That distinction is the whole reason vaccination gives long-term immunity.
“A vaccine contains a weakened pathogen or its antigens. The antigens stimulate lymphocytes, which produce antibodies with a complementary shape, and also memory cells. If the real pathogen arrives later, the memory cells produce antibodies faster and in greater quantity, so it is destroyed before symptoms develop.”
Clean drinking water closes one route, but the villagers are still releasing untreated sewage containing the bacterium. It reaches the river, and from there it reaches food washed in the river, hands, and the next village downstream.
Sewage treatment stops the bacterium being released, but the villagers are still drinking water that other people, and other villages, have contaminated. The source is upstream of them.
Cholera transmission is a loop: from an infected person’s faeces, into the water, into the next person’s mouth. Each measure alone leaves one half of the loop open, so the loop still runs. If each measure simply removed its own share of cases, you would expect 0.40 × 0.45 = 0.18 of the cases to be left, an 82 % fall. The real fall, 97 %, is bigger than that: closing both halves of the loop stops the bacterium going round it at all, instead of slowing it down.
This is why the syllabus lists a clean water supply and sewage treatment as two separate measures, and why an answer naming both is stronger than an answer naming one twice in different words.
The Night-Before Checklist
The definition of a pathogen. The definition of a transmissible disease. Direct transmission, and the phrase “blood and other body fluids”. The four indirect routes: surfaces, food, animals, air. The five body defences, and the fact that the list is complete. Which white blood cell does phagocytosis and which produces antibodies. The five control measures, each with a reason. The definition of active immunity. Where an antigen is and who makes an antibody. The word complementary. The two things an antibody causes to happen after it binds. The four steps of vaccination, ending with memory cells. Why the secondary response is faster, larger and longer. Why vaccinating most of a population protects the rest. The definition of passive immunity, in three clauses. The two natural routes for it. Why breastfeeding matters. Why no memory cells are produced. Cholera: bacterium, contaminated water, toxin, chloride ions, osmosis, diarrhoea, dehydration, loss of ions.
That list is the entire topic. If you can produce it out loud in four minutes, you are ready.