What you'll be able to do by the end
- Define health and explain the difference between communicable and non-communicable disease
- Explain how diseases interact with each other
- Describe the effects of lifestyle factors on non-communicable disease
- Describe named communicable diseases in humans and plants
- Explain the body's defence systems
- Explain how white blood cells defend the body
- Explain vaccination and herd immunity
- Explain the difference between antibiotics, antivirals and painkillers
- Describe the development and testing of new drugs
- Describe the role of enzymes and the digestive system
Part 1 — Health and disease
Health is a state of physical and mental wellbeing — not merely the absence of disease. Mental health is health.
Two categories of disease
| Communicable | Non-communicable | |
|---|---|---|
| Spread? | Yes — by pathogens | No |
| Caused by | Bacteria, viruses, fungi, protists | Genetics, lifestyle, environment |
| Examples | Measles, HIV, tuberculosis, malaria | Cancer, heart disease, diabetes, asthma |
Other factors affecting health
- Diet — deficiency causes specific diseases; excess causes others
- Stress — affects both mental and physical health
- Life situation — access to healthcare, housing, clean water, income
Diseases interact
This is a favourite exam point, and it's four specific interactions:
| Interaction | Example |
|---|---|
| A weakened immune system makes communicable disease more likely | People with HIV are far more vulnerable to tuberculosis |
| Viruses can trigger cancers | HPV infection can lead to cervical cancer |
| Immune reactions can trigger allergies | A pathogen can leave a person with skin rashes or asthma |
| Physical illness can cause mental illness | Long-term or severe physical ill health frequently leads to depression |
Diseases are not independent of one another. One can cause, worsen or enable another. Questions often present a scenario and ask you to explain the link.
Part 2 — Non-communicable disease
Risk factors
A risk factor is anything that increases the chance of developing a disease. It does not guarantee it.
| Risk factor | Increases the risk of |
|---|---|
| Smoking | Lung disease, lung cancer, heart disease, stroke |
| Obesity | Type 2 diabetes, heart disease, some cancers |
| Alcohol | Liver disease, brain function damage |
| High-fat diet | Cardiovascular disease |
| Lack of exercise | Obesity, heart disease |
| Carcinogens — ionising radiation, tar, asbestos | Cancer |
| Alcohol or smoking in pregnancy | Damage to the developing fetus |
A correlation means two things vary together. It does not prove one causes the other.
To establish causation, scientists need a plausible mechanism, repeated studies, and evidence that the effect increases with dose. Exam questions frequently show correlational data and ask whether it proves causation. The answer is no, and you must say why.
Cancer
Cancer is the result of changes in cells leading to uncontrolled growth and division.
| Type | Behaviour |
|---|---|
| Benign tumour | Contained in one place, usually in a membrane. Does not invade other tissues. Not cancerous |
| Malignant tumour | Invades neighbouring tissues; cells can spread in the blood to form secondary tumours. This is cancer |
Risk factors: smoking, obesity, UV exposure, viral infection, ionising radiation, some inherited genes.
Part 3 — Communicable disease
A pathogen is a microorganism that causes infectious disease.
The four types of pathogen
| Type | Reproduction | Damage caused |
|---|---|---|
| Bacteria | Divide rapidly | Produce toxins that damage tissue |
| Viruses | Live and reproduce inside cells | The cell bursts, releasing more viruses |
| Protists | Often transferred by a vector | Damage cells directly |
| Fungi | Grow and spread | Damage tissue |
How pathogens spread
| Route | Examples |
|---|---|
| Direct contact | Athlete's foot, some STIs |
| Water | Cholera |
| Air (droplets) | Influenza, measles, tuberculosis |
| Vectors | Malaria (mosquito), some plant diseases (insects) |
| Body fluids | HIV |
The named diseases
| Disease | Pathogen | Spread | Symptoms | Prevention or treatment |
|---|---|---|---|---|
| Measles | Virus | Droplets from coughs and sneezes | Fever, red skin rash; can be fatal | Vaccination in childhood |
| HIV | Virus | Sexual contact, exchange of body fluids, shared needles | Flu-like initially; later attacks immune cells | Antiretroviral drugs; barrier contraception |
| Tobacco mosaic virus | Virus | Contact between plants | Mosaic pattern of discolouration on leaves; reduced photosynthesis and growth | Remove infected plants; hygiene |
| Rose black spot | Fungus | Wind and water | Purple or black spots on leaves; leaves fall; reduced photosynthesis | Fungicide; remove and destroy affected leaves |
| Malaria | Protist | Mosquito vector | Recurrent fever; can be fatal | Mosquito nets, insect repellent, preventing mosquito breeding |
| Salmonella | Bacterium | Contaminated food | Fever, cramps, vomiting, diarrhoea | Poultry vaccination; food hygiene |
| Gonorrhoea | Bacterium | Sexual contact | Thick yellow-green discharge, pain on urination | Antibiotics; barrier contraception |
HIV is the virus. AIDS is the late-stage condition that develops when HIV has damaged the immune system so badly that the body can no longer deal with other infections.
With antiretroviral treatment, a person with HIV may never develop AIDS.
Reducing the spread — four methods
- Hygiene — handwashing, disinfecting surfaces, food preparation practice
- Isolating infected individuals — reduces contact
- Destroying vectors — insecticides, removing standing water
- Vaccination — reduces the number of susceptible people
Part 4 — Plant disease
Plants become diseased too, and detecting it is worth marks.
Signs of disease in plants
- Stunted growth
- Spots on leaves
- Areas of decay or rot
- Growths
- Malformed stems or leaves
- Discolouration
- Presence of visible pests
Identifying the cause
- Reference to a gardening manual or website
- Taking infected samples to a laboratory for identification
- Using testing kits containing monoclonal antibodies
Mineral deficiencies
| Deficiency | Effect | Why |
|---|---|---|
| Nitrate | Stunted growth | Nitrate ions are needed to make amino acids and therefore proteins |
| Magnesium | Chlorosis — yellow leaves | Magnesium is needed to make chlorophyll |
Physical and chemical plant defences
| Physical | Chemical | Mechanical |
|---|---|---|
| Cellulose cell walls | Antibacterial chemicals | Thorns and hairs |
| Tough waxy cuticle | Poisons to deter herbivores | Leaves that droop or curl when touched |
| Layers of dead cells (bark) | Mimicry to trick animals |
Part 5 — The human defence systems
Non-specific defences — the first line
These stop pathogens entering, and they work against everything.
| Defence | How it works |
|---|---|
| Skin | A physical barrier; produces antimicrobial secretions; scabs seal wounds |
| Nose | Hairs and mucus trap particles |
| Trachea and bronchi | Secrete mucus to trap pathogens; cilia waft mucus up to the throat to be swallowed |
| Stomach | Hydrochloric acid kills pathogens in food and in swallowed mucus |
Smoking damages or destroys the cilia. Mucus and trapped pathogens are no longer removed, which is why smokers cough and are more prone to chest infections. That's a two-mark link between two parts of the specification.
White blood cells — the second line
If a pathogen gets in, white blood cells respond in three ways:
| Action | What happens |
|---|---|
| Phagocytosis | The white blood cell engulfs and digests the pathogen |
| Producing antibodies | Proteins that bind to specific antigens on the pathogen, clumping them together and marking them for destruction |
| Producing antitoxins | Neutralise the toxins produced by bacteria |
Antigen — a marker molecule on the surface of a pathogen. Antibody — a protein made by white blood cells that binds to a specific antigen. Antitoxin — neutralises a toxin.
Antibodies are specific. An antibody for measles will not work against influenza — the antigens are different shapes.
Part 6 — Vaccination
How it works
- A small quantity of dead or inactive pathogen is introduced into the body
- White blood cells recognise the antigens and produce antibodies
- Memory cells remain in the body afterwards
- If the same pathogen enters later, antibodies are produced much more rapidly and in greater quantity
- The pathogen is destroyed before the person becomes ill
"The vaccine contains antibodies." It doesn't. It contains antigens — dead or inactive pathogen. Your body makes the antibodies.
"The vaccine kills the pathogen." It doesn't. It trains the immune system to respond faster next time.
Herd immunity
If a large proportion of the population is vaccinated, the spread of the pathogen is greatly reduced. This protects people who cannot be vaccinated — babies too young, people with certain medical conditions, people receiving immunosuppressant treatment.
If vaccination rates fall below the threshold, herd immunity is lost and outbreaks return. This is why measles has reappeared in populations where it had almost disappeared.
The arguments
| For | Against |
|---|---|
| Prevents serious illness and death | Vaccines are never 100% effective |
| Has eliminated or greatly reduced many diseases | A small number of people have adverse reactions |
| Protects the vulnerable through herd immunity | Some object on personal or religious grounds |
| Reduces cost and pressure on health services |
This is an evaluate question and students should be able to state the arguments on both sides accurately.
Adverse reactions are real but rare, and the risk from the disease is very much higher than the risk from the vaccine. Present the actual comparison rather than dismissing the concern — a student who has heard the objection at home will disengage from a lesson that pretends it doesn't exist.
Part 7 — Drugs and medicines
The three types
| Type | What it does | Works on |
|---|---|---|
| Antibiotic | Kills bacteria inside the body | Bacteria only |
| Antiviral | Slows or stops viral reproduction | Viruses |
| Painkiller | Treats the symptoms — relieves pain | Neither — it does not kill the pathogen |
Antibiotics do not work on viruses.
Viruses live and reproduce inside your own cells. A drug that killed them would have to damage the host cell too, which is why antiviral drugs are so difficult to develop. This is why taking antibiotics for a cold is useless — and worse than useless, because it drives resistance.
Antibiotic resistance
Bacteria evolve resistance through natural selection:
- A random mutation makes some bacteria resistant
- The antibiotic kills the non-resistant bacteria
- The resistant ones survive and reproduce
- The resistant population increases
Slowing it:
- Doctors should not prescribe antibiotics for viral infections or non-serious conditions
- Patients must complete the full course
- Restrict agricultural use
Antibiotics have greatly reduced deaths from bacterial disease. Resistant strains threaten to reverse that, and developing new antibiotics is slow and expensive.
Where drugs come from
| Drug | Origin |
|---|---|
| Digitalis | Foxglove plants — heart conditions |
| Aspirin | Willow bark — painkiller |
| Penicillin | Penicillium mould, discovered by Alexander Fleming |
Most new drugs are now synthesised by chemists, though the starting point may still be a natural compound.
Testing new drugs
Drugs are tested for toxicity, efficacy and dose.
| Stage | What happens |
|---|---|
| Preclinical — cells and tissues | Testing on cells, tissues and live animals in a laboratory |
| Clinical — healthy volunteers | Very low doses, to check for safety and side effects |
| Clinical — patients | Testing on patients with the condition, finding the optimum dose and testing efficacy |
| Peer review and publication | Results scrutinised by other scientists before approval |
Placebos and blind trials
A placebo looks identical to the drug but contains no active ingredient. It controls for the psychological effect of believing you are being treated.
| Trial type | Who knows |
|---|---|
| Blind | The patient does not know which they received |
| Double blind | Neither the patient nor the doctor knows until the trial ends |
Double blind trials prevent bias in how the doctor assesses and reports the patient's response.
Placebos are not used where withholding treatment would harm the patient. In serious conditions, a new drug is compared against the existing best treatment, not against nothing.
Part 8 — Enzymes and digestion
What enzymes are
Enzymes are biological catalysts — they speed up reactions without being used up. They are proteins, folded into a shape with an active site where the substrate binds.
The lock and key model: the substrate fits the active site exactly, like a key in a lock. The enzyme is specific to one substrate.
Factors affecting enzyme action
| Factor | Effect |
|---|---|
| Temperature | Rate increases with temperature up to an optimum. Above it, the enzyme denatures — the active site changes shape and the substrate no longer fits |
| pH | Each enzyme has an optimum pH. Outside it, the enzyme denatures |
| Substrate concentration | Rate increases until all active sites are occupied, then plateaus |
Enzymes are not alive. Denaturing is a permanent change in shape. "The enzyme dies" loses the mark every single time.
Digestive enzymes
| Enzyme | Breaks down | Into | Made in |
|---|---|---|---|
| Amylase | Starch | Sugars | Salivary glands, pancreas, small intestine |
| Protease | Proteins | Amino acids | Stomach, pancreas, small intestine |
| Lipase | Lipids (fats) | Fatty acids and glycerol | Pancreas, small intestine |
Bile
Made in the liver, stored in the gall bladder, released into the small intestine.
Two jobs:
- Neutralises stomach acid, providing the alkaline conditions enzymes in the small intestine need
- Emulsifies fats — breaks large fat droplets into smaller ones, increasing the surface area for lipase to work on
It does not break chemical bonds. It changes the physical state of the fat and the pH of the environment. Calling bile an enzyme is a common and costly error.
Food tests
| Test | Reagent | Positive result |
|---|---|---|
| Starch | Iodine solution | Orange-brown → blue-black |
| Sugars | Benedict's solution, heated | Blue → green, yellow, orange or brick red |
| Protein | Biuret solution | Blue → purple |
| Lipids | Ethanol, then water | Clear → cloudy white emulsion |
Exam-style questions
Answer all questions. Marks are shown in brackets. Total: 81 marks.
Question 1
(Total 8 marks)
Question 2
(Total 7 marks)
Question 3
(Total 11 marks)
Question 4
(Total 11 marks)
Question 5
(Total 10 marks)
Question 6
(Total 11 marks)
Question 7
(Total 10 marks)
Question 8
(Total 13 marks)
TOTAL FOR PAPER: 81 MARKS
Mark scheme
Question 1
| (a) 2. A state of physical and mental wellbeing | 1 |
| Not merely the absence of disease | 1 |
| (b) 2. Communicable diseases can be spread; non-communicable cannot | 1 |
| Communicable are caused by pathogens; non-communicable by genetics, lifestyle or environment | 1 |
| (c) 2. A defective immune system responds less effectively to pathogens | 1 |
| So the person is more likely to develop infectious disease | 1 |
(d) 2 — any one, explained: viruses living in cells can trigger cancers (2) · immune reactions can trigger allergies such as skin rashes or asthma (2) · severe physical illness can lead to depression or other mental illness (2).
Question 2
| (a) 2. Something that increases the chance of developing a disease | 1 |
| But does not guarantee it | 1 |
| (b) 3. A correlation shows the two vary together, not that one causes the other | 1 |
| Another factor may be responsible — people who eat processed meat may differ in other ways | 1 |
| Causation requires a plausible mechanism and repeated studies showing the effect increases with dose | 1 |
(c) 2 — benign tumours stay in one place and do not invade other tissues (1); malignant tumours invade neighbouring tissue and can spread in the blood to form secondary tumours (1).
Question 3
(a) 4 — Measles: virus · Malaria: protist · Rose black spot: fungus · Gonorrhoea: bacterium.
| (b) 3. Spread by mosquitoes acting as vectors | 1 |
| Prevented by mosquito nets | 1 |
| And by preventing mosquitoes breeding, e.g. removing standing water, or insecticides | 1 |
| (c) 2. The virus reproduces inside the cell | 1 |
| The cell bursts, releasing more viruses and damaging the tissue | 1 |
| (d) 2. HIV is the virus | 1 |
| AIDS is the late-stage condition when the immune system is so badly damaged it can no longer fight other infections | 1 |
Question 4
(a) 3 — any three: skin as a physical barrier · nose hairs and mucus trap particles · cilia and mucus in the trachea · stomach acid kills pathogens.
| (b) 3. Phagocytosis, engulfing and digesting pathogens | 1 |
| Producing antibodies | 1 |
| Producing antitoxins | 1 |
| (c) 2. Antibodies are specific to a particular antigen | 1 |
| Measles and influenza have differently shaped antigens, so the antibody will not bind | 1 |
| (d) 3. Smoking damages or destroys the cilia in the airways | 1 |
| Mucus containing trapped pathogens is no longer moved out of the lungs | 1 |
| Pathogens remain in the airways, so infections are more likely | 1 |
Question 5
| (a) 4. A small quantity of dead or inactive pathogen is introduced | 1 |
| White blood cells recognise the antigens and produce antibodies | 1 |
| Memory cells remain in the body | 1 |
| If the same pathogen enters later, antibodies are produced much faster and in greater quantity, destroying it before illness develops | 1 |
| (b) 3. The vaccine contains antigens — dead or inactive pathogen — not antibodies | 1 |
| The body produces its own antibodies in response | 1 |
| The vaccine does not kill the pathogen; it prepares the immune system to respond rapidly if infected later | 1 |
| (c) 3. If a large proportion of the population is vaccinated, the pathogen cannot spread easily | 1 |
| This protects unvaccinated people because they are less likely to encounter it | 1 |
| It protects those who cannot be vaccinated — babies, or people with certain medical conditions | 1 |
Question 6
| (a) 3. Antibiotics kill bacteria; influenza is caused by a virus | 1 |
| Viruses reproduce inside the body's own cells | 1 |
| A drug that killed the virus would damage the host cell, which is why antivirals are difficult to develop | 1 |
| (b) 2. An antibiotic kills the pathogen and cures the disease | 1 |
| A painkiller only relieves the symptoms and does not kill the pathogen | 1 |
| (c) 4. A random mutation makes some bacteria resistant | 1 |
| The antibiotic kills the non-resistant bacteria | 1 |
| The resistant bacteria survive and reproduce | 1 |
| The resistant population increases over time | 1 |
(d) 2 — any two: doctors should not prescribe antibiotics for viral infections or minor conditions · patients should complete the full course · agricultural use should be restricted.
Question 7
(a) 3 — toxicity · efficacy · dose.
| (b) 4. Preclinical testing on cells and tissues | 1 |
| Then on live animals in a laboratory | 1 |
| Clinical trials on healthy volunteers at very low doses to check for safety | 1 |
| Then on patients with the condition, to find the optimum dose and test efficacy | 1 |
| (c) 3. Neither the patient nor the doctor knows who received the drug and who received the placebo | 1 |
| This prevents the patient's expectations affecting the result | 1 |
| And prevents bias in how the doctor assesses and records the patient's response | 1 |
Question 8
| (a) 2. A biological catalyst | 1 |
| That speeds up a reaction without being used up | 1 |
| (b) 3. The enzyme denatures | 1 |
| The shape of the active site changes | 1 |
| The substrate no longer fits, so the reaction stops | 1 |
Do not credit "the enzyme dies."
(c) 2 — protease (1), producing amino acids (1).
| (d) 4. Neutralises stomach acid | 1 |
| Providing the alkaline conditions enzymes in the small intestine need | 1 |
| Emulsifies fats into smaller droplets | 1 |
| Increasing the surface area for lipase to act on, so digestion is faster | 1 |
| (e) 2. Bile does not break chemical bonds or act as a catalyst | 1 |
| It changes the physical state of the fat and the pH of the environment | 1 |
Common mistakes in this topic
"Antibiotics kill viruses." They do not, and this is on every paper.
"The vaccine contains antibodies." It contains antigens. Your body makes the antibodies.
"The enzyme dies." Enzymes are not alive. They denature.
"Bile is an enzyme." It emulsifies and neutralises. It does not catalyse.
Confusing antigen, antibody and antitoxin. Marker, protein that binds to it, toxin neutraliser.
Saying a correlation proves cause. It never does on its own.
Confusing HIV and AIDS. One is the virus, one is the late-stage condition.
Naming a white blood cell action without describing it. "Phagocytosis" alone is often not enough — say it engulfs and digests.
Teaching notes
The antibiotics-and-viruses point should be taught early and returned to constantly. It appears on every paper, it's widely misunderstood outside school, and understanding why — viruses reproduce inside your own cells — is what makes it stick rather than being another fact.
Antigen, antibody, antitoxin need drilling as a set. Three similar words, three different meanings, and students who confuse them lose marks across the whole immunity topic.
Teach vaccination honestly, including the objections. A student who has heard vaccine scepticism at home will disengage from a lesson that pretends the concern doesn't exist. State the arguments accurately, give the actual comparison of risks, and let the evidence do the work.
Correlation and causation deserves a lesson of its own. It is examined directly, it appears in every science subject, and it is one of the few things on the specification that students will use for the rest of their lives.
The cilia and smoking link is a two-mark gift. It connects the defence systems to the lifestyle risk factors, and students who spot the connection tend to spot others.
Enzymes underpin the whole subject. Digestion, respiration, photosynthesis and homeostasis all depend on them. Time spent on denaturing here saves confusion in four other topics.
Written for GCSE Biology, all major UK boards. Check your specification — monoclonal antibodies, plant disease detail and some drug development content vary by board and tier. Reviewed 2026.