HookThe country doctor who beat a disease he could not see
In the summer of 1796, a Gloucestershire country doctor named Edward Jenner acted on a piece of dairy-farm folklore: milkmaids who caught cowpox seemed never to catch smallpox, the disease then killing perhaps one European in ten. Jenner scraped pus from a milkmaid's cowpox sore, scratched it into the arm of an eight-year-old boy, James Phipps, and weeks later deliberately exposed him to smallpox. The boy stayed well. Jenner had no idea what a virus or an antibody was — germ theory was still decades away — yet he had invented vaccination, from the Latin vacca, cow.
It worked so well that in 1980 the World Health Organization declared smallpox eradicated — the only human disease ever wiped from the planet, and entirely by vaccination. B3 is the biology Jenner was missing: what pathogens are and how they spread, the specific diseases you must be able to name, the defences your body throws up, how vaccines and antibiotics turn the fight in our favour, and how new drugs — and the monoclonal antibodies now used to diagnose and treat disease — are discovered and tested.
ModelPathogens and how disease spreads
Communicable (infectious) diseases are caused by pathogens — microorganisms that pass from one host to another. There are four types: bacteria, viruses, protists and fungi. They spread in three main ways: through the air (droplets from coughs and sneezes), through water, and by direct contact — including contaminated surfaces and vectors such as insects.
Bacteria and viruses make you ill in different ways, and the distinction earns marks. Bacteria reproduce rapidly inside the body and can release poisons — toxins — that damage your tissues and make you feel unwell. Viruses invade your own cells and hijack them, forcing each cell to make thousands of new virus particles until it bursts; that cell damage is what causes the symptoms.
Because most infections cannot be cured instantly, controlling spread is central to fighting them: hygiene such as handwashing and disinfectants, isolating infected individuals, destroying vectors, and vaccinating the population. Every named disease that follows is really an exercise in three questions — which type of pathogen, spread how, and stopped how.
CaseThe diseases you must be able to name
Viral. Measles spreads by droplets and causes fever and a red rash; it can be fatal, which is why the MMR vaccine is given in childhood. HIV spreads in bodily fluids — through sexual contact or shared needles — and attacks the immune system's white blood cells; untreated it leads to AIDS, but antiretroviral drugs now hold it in check. Tobacco mosaic virus (TMV) is a plant virus that mottles and discolours leaves, cutting photosynthesis and stunting growth.
Bacterial. Salmonella causes food poisoning from undercooked or contaminated food — fever, cramps, vomiting and diarrhoea, all from toxins — and UK poultry are vaccinated to control it. Gonorrhoea is a sexually transmitted infection giving a thick yellow discharge and pain on urinating; once easily cured with penicillin, its spread is now limited by barrier contraception because resistant strains have appeared.
Fungal and protist. Rose black spot is a fungus causing purple-black spots on rose leaves, which then yellow and drop, reducing photosynthesis; it spreads by water and wind and is treated with fungicides or by removing affected leaves. Malaria is caused by a protist, Plasmodium, carried by mosquitoes acting as vectors; it brings recurrent fevers and is often fatal, and is controlled by stopping mosquitoes breeding and by nets that prevent bites.
MechanismYour body's defences
The first line of defence is non-specific and largely physical or chemical. The skin is a barrier that also scabs over to reseal breaks. The nose has hairs and mucus that trap particles. The trachea and bronchi are lined with mucus and tiny hairs called cilia that sweep trapped pathogens up and away from the lungs. And the stomach's hydrochloric acid kills most pathogens in swallowed food and mucus.
If a pathogen gets past all that, white blood cells take over in three ways. In phagocytosis, a white blood cell engulfs and digests the pathogen. In antibody production, other white blood cells make antibodies whose shape locks onto a specific antigen — a marker molecule on the pathogen's surface — flagging it for destruction. And antitoxins are produced to neutralise the toxins that bacteria release.
The crucial property is that antibodies are specific: one shape fits one antigen, like a key to a lock. That specificity is the whole basis of both natural immunity and vaccination — because once your body has made a particular antibody, it can make it again far faster the next time.
MechanismVaccination and herd immunity
A vaccine contains small quantities of dead or inactive forms of a pathogen, carrying its antigens. Injected, they trigger the white blood cells to produce the matching antibodies — without you becoming ill. Crucially, some of those cells persist afterwards as memory cells.
If the real pathogen later invades, the memory response is fast and large: antibodies flood out before the pathogen can multiply enough to make you ill. That is why a second exposure to an antigen produces a much bigger, quicker antibody spike than the first — a difference examiners love to show on a graph as the primary versus the secondary response.
Vaccinate enough of a population and even the unvaccinated are protected, because the pathogen cannot find enough susceptible hosts to keep spreading — this is herd immunity. Exactly how high 'enough' is depends on how contagious the disease is, and that can be worked out.
Measles is extraordinarily infectious: on average one case would infect about 15 others in a fully susceptible population — a basic reproduction number \(R_0 \approx 15\). The fraction of people who must be immune to stop sustained spread is \[1 - \dfrac{1}{R_0} = 1 - \dfrac{1}{15} \approx 0.93.\] So roughly 93 to 95% must be vaccinated — which is exactly why public-health bodies chase a 95% MMR target, and why measles returns the moment coverage slips below it. Compare a less contagious disease with \(R_0 \approx 4\): the threshold is only \(1 - 1/4 = 0.75\), or 75%. One line of arithmetic explains why some diseases demand near-total coverage while others do not.
CaseAntibiotics, painkillers and the resistance crisis
The first antibiotic, penicillin, was discovered by Alexander Fleming in 1928 when a stray Penicillium mould killed the bacteria on a culture plate he had left out over a holiday. Antibiotics kill bacteria inside the body and have hugely reduced deaths from bacterial disease; each one is specific to certain bacteria.
Crucially, antibiotics do not kill viruses, because viruses reproduce inside your own cells where a drug cannot reach them without harming you too. That is why antibiotics do nothing for a cold or the flu. Painkillers and similar drugs, such as paracetamol, treat the symptoms — they make you feel better — but they kill no pathogens at all.
Then comes the problem: antibiotic resistance. Bacteria mutate at random; a few happen to survive an antibiotic, then reproduce and pass on the resistance — and overuse and unfinished courses speed this up. Resistant strains such as MRSA are very hard to treat, so doctors now prescribe antibiotics less freely and patients are told to finish every course. It is natural selection (B6) playing out inside a hospital ward.
MechanismDiscovering and testing new drugs
Many drugs began in nature. Digitalis, a heart drug, comes from foxgloves; aspirin originates in a chemical from willow bark; penicillin came from Fleming's Penicillium mould. Most modern drugs are now synthesised by chemists, but the original leads often came from plants and microorganisms.
A new drug must be safe, effective, stable, and able to be taken into and cleared from the body — and it is tested in stages. Preclinical testing uses cells, then tissues, then live animals to check toxicity and rough dose. Clinical trials then begin on healthy volunteers at very low doses to test safety, before moving to patients to find the optimum dose and check that the drug actually works.
Trials are made rigorous with placebos and a double-blind design — neither the patient nor the doctor knows who received the real drug — so that expectations cannot skew the result. Finally the findings are peer-reviewed before publication, to catch errors and stop false claims reaching patients.
Suppose a new asthma inhaler is being trialled. Stage 1 (preclinical): it is tested on cultured lung cells and then on animals for toxicity — and passes. Stage 2 (clinical, safety): 50 healthy volunteers take tiny doses; there is no serious harm, so the dose is raised gradually to map the safe range. Stage 3 (clinical, efficacy): 500 asthma patients are split at random — half get the drug, half an identical-looking placebo, and neither they nor the assessing doctors know which (double-blind) — and lung function is compared. Only if the drug group improves significantly more than the placebo group, and the work survives peer review, does it reach patients. Each stage answers a different question in order: first 'is it poisonous?', then 'is it safe in people?', then 'does it beat doing nothing?'.
MechanismMonoclonal antibodies
A monoclonal antibody is a single, identical type of antibody, specific to one binding site and produced in bulk. Making them is a Higher-tier sequence: inject a mouse with the chosen antigen so its lymphocytes make the right antibody; because those lymphocytes do not divide well, fuse each with a tumour cell to make a hybridoma — a cell that both divides endlessly and makes the antibody; then clone the useful hybridoma and collect its antibodies.
Their uses turn on that specificity. Pregnancy tests use monoclonal antibodies that bind the hormone hCG in urine, colouring a line. In laboratories they measure the levels of hormones and other chemicals in blood and detect pathogens. In research they bind to one specific molecule in a tissue sample and are tagged with a fluorescent dye, so the molecule's location lights up under the microscope.
In treatment, a monoclonal antibody can be built to bind only to a cancer cell's antigens and to carry a drug, a radioactive substance or a toxin directly to it, sparing healthy cells. The early hope was a 'magic bullet'; in practice they caused more side effects than expected, so they are used more narrowly than first imagined — which is itself a fair evaluation point in an exam answer.
ModelPlant disease — detection and defence
Spotting a sick plant relies on visible signs: stunted growth, spots on the leaves, patches of decay, abnormal growths, malformed stems or leaves, discolouration, and the presence of pests. To identify the cause you might consult a gardening manual or website, send the plant to a laboratory to identify the pathogen, or use a monoclonal-antibody test kit.
Not every plant problem is a pathogen, though. Mineral-ion deficiencies mimic disease: too little nitrate stunts growth, because the plant cannot make enough proteins; too little magnesium causes chlorosis — yellow leaves — because magnesium is needed to make chlorophyll.
Plants defend themselves in three ways. Physical defences include cellulose cell walls, a tough waxy cuticle, and layers of dead bark. Chemical defences include antibacterial compounds and poisons that deter herbivores — some of which we now use as human medicines. Mechanical defences include thorns and hairs that stop animals eating the plant, leaves that droop or curl when touched, and mimicry — looking like another organism, or like an already-unhealthy plant, to fool pests into leaving it alone.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
For the named diseases, examiners want a tight package each time: the type of pathogen, one or two symptoms, one way it spreads, and one way that spread is prevented. Keep antigen and antibody straight — the antigen is the marker on the pathogen, the antibody is your protein that locks onto it — because muddling them costs marks in almost every immunity question.
Describe vaccination as an ordered sequence — inactive pathogen, then white blood cells make antibodies, then memory cells form, then a fast large response on reinfection — rather than a vague 'it makes you immune'. On antibody-response graphs, name the primary and secondary responses and compare their size and speed. Treat antibiotic-resistance questions as natural-selection questions in disguise: mention random mutation, survival of resistant bacteria, and their reproduction.
Drug-testing answers must keep the stages in order and attach the right purpose to each — preclinical for toxicity, healthy volunteers for safety, patients for efficacy and dose — and name placebo, double-blind and peer review. For the Higher-tier monoclonal production, learn the chain 'mouse lymphocyte, fuse with tumour cell, hybridoma, clone, collect', and be ready to evaluate why monoclonal antibodies ended up used more narrowly than first hoped.