B10: Health and Disease
Communicable and non-communicable diseases, risk factors, cancer, and correlation vs causation
Key Definitions
Health — A state of physical and mental well-being, not merely the absence of disease or infirmity (WHO definition).
Communicable disease — A disease that can be transmitted (passed) from one person to another; caused by pathogens (bacteria, viruses, fungi, protists).
Non-communicable disease — A disease that cannot be passed from person to person; caused by lifestyle factors, genetics, or environmental factors (e.g. heart disease, cancer, diabetes).
Risk factor — Any aspect of a person's lifestyle or environment that increases their chance of developing a disease.
Cancer — A disease caused by the uncontrolled division of abnormal cells, which can form tumours.
Communicable disease — A disease that can be transmitted (passed) from one person to another; caused by pathogens (bacteria, viruses, fungi, protists).
Non-communicable disease — A disease that cannot be passed from person to person; caused by lifestyle factors, genetics, or environmental factors (e.g. heart disease, cancer, diabetes).
Risk factor — Any aspect of a person's lifestyle or environment that increases their chance of developing a disease.
Cancer — A disease caused by the uncontrolled division of abnormal cells, which can form tumours.
Communicable vs Non-communicable Diseases
| Feature | Communicable Disease | Non-communicable Disease |
|---|---|---|
| Can be passed between people? | Yes | No |
| Cause | Pathogens (bacteria, viruses, fungi, protists) | Lifestyle, genetics, environment, ageing |
| Duration | Usually short-term (but can be chronic, e.g. HIV) | Usually long-term or lifelong |
| Examples | Measles, malaria, tuberculosis, influenza | Coronary heart disease, Type 2 diabetes, cancer, asthma |
| Treatment | Antibiotics (bacterial), antivirals, vaccination | Lifestyle changes, medication, surgery |
Diseases can interact:
— A person with a communicable disease may be more susceptible to a non-communicable disease. For example, HIV weakens the immune system, making the person more vulnerable to infections and certain cancers.
— A person with a non-communicable disease may be more susceptible to a communicable disease. For example, a person with lung disease from smoking has damaged lung tissue, making them more vulnerable to infections like pneumonia and tuberculosis.
— Physical illness can also affect mental health. A person diagnosed with cancer may experience depression or anxiety, which can reduce their ability to cope with the physical disease.
— A person with a communicable disease may be more susceptible to a non-communicable disease. For example, HIV weakens the immune system, making the person more vulnerable to infections and certain cancers.
— A person with a non-communicable disease may be more susceptible to a communicable disease. For example, a person with lung disease from smoking has damaged lung tissue, making them more vulnerable to infections like pneumonia and tuberculosis.
— Physical illness can also affect mental health. A person diagnosed with cancer may experience depression or anxiety, which can reduce their ability to cope with the physical disease.
Risk Factors for Non-communicable Diseases
Risk factors are aspects of a person's lifestyle or body that increase the likelihood of developing a disease:
Smoking — Increases risk of lung cancer, cardiovascular disease, COPD, stroke. Contains carcinogens (cancer-causing chemicals) and carbon monoxide (reduces oxygen in blood). Damages artery walls, promoting atheroma formation.
Alcohol — Increases risk of liver cirrhosis, liver cancer, brain damage, heart disease. Alcohol is toxic and damages liver cells over time.
Obesity — Increases risk of Type 2 diabetes, cardiovascular disease, certain cancers, joint problems. Excess body fat affects hormone levels and metabolism.
Lack of exercise — Increases risk of cardiovascular disease, Type 2 diabetes, obesity, osteoporosis. Regular exercise strengthens the heart, improves circulation, and helps maintain a healthy weight.
UV exposure — Increases risk of skin cancer. UV radiation damages DNA in skin cells, which can lead to mutations and uncontrolled cell division.
Smoking — Increases risk of lung cancer, cardiovascular disease, COPD, stroke. Contains carcinogens (cancer-causing chemicals) and carbon monoxide (reduces oxygen in blood). Damages artery walls, promoting atheroma formation.
Alcohol — Increases risk of liver cirrhosis, liver cancer, brain damage, heart disease. Alcohol is toxic and damages liver cells over time.
Obesity — Increases risk of Type 2 diabetes, cardiovascular disease, certain cancers, joint problems. Excess body fat affects hormone levels and metabolism.
Lack of exercise — Increases risk of cardiovascular disease, Type 2 diabetes, obesity, osteoporosis. Regular exercise strengthens the heart, improves circulation, and helps maintain a healthy weight.
UV exposure — Increases risk of skin cancer. UV radiation damages DNA in skin cells, which can lead to mutations and uncontrolled cell division.
Example 1: How Smoking Increases Disease Risk
Cigarette smoke contains over 70 known carcinogens (cancer-causing chemicals) that damage DNA in lung cells, increasing the risk of lung cancer. Carbon monoxide in smoke binds to haemoglobin, reducing the blood's oxygen-carrying capacity, which strains the heart and increases cardiovascular disease risk. Chemicals in smoke also damage the lining of arteries, promoting fatty deposit buildup (atheroma) and narrowing of coronary arteries.
Example 2: How Obesity Increases Type 2 Diabetes Risk
Excess body fat, particularly around the abdomen, makes body cells less responsive to insulin (insulin resistance). The pancreas produces more insulin to compensate, but over time it cannot keep up, and blood glucose levels remain high. This leads to Type 2 diabetes. Losing weight through diet and exercise can improve insulin sensitivity and reduce the risk.
Cancer: Benign vs Malignant Tumours
| Feature | Benign Tumour | Malignant Tumour |
|---|---|---|
| Growth rate | Slow | Rapid |
| Cell division | Cells divide but stop at a certain size | Cells divide uncontrollably |
| Spread | Contained within a fibrous capsule; does NOT spread to other parts of the body | Breaks through the capsule; cells break off and spread to other parts of the body (metastasis) |
| Danger | Usually not life-threatening (unless pressing on vital organs) | Life-threatening; can damage organs and spread |
| Treatment | Usually removed by surgery alone | Requires surgery, chemotherapy, and/or radiotherapy |
| Recurrence | Unlikely to return after removal | Can return after treatment; may spread to new sites |
How cancer develops:
1. Mutation — Changes in the DNA of a cell cause it to divide uncontrollably.
2. Tumour formation — The rapidly dividing cells form a mass of abnormal cells (a tumour).
3. Malignant tumours can spread (metastasise) when cells break off and travel through the blood or lymph to other parts of the body, where they form secondary tumours.
Carcinogens — Substances that increase the risk of mutations and cancer. Examples: tobacco smoke, UV radiation, ionising radiation, asbestos, certain chemicals.
1. Mutation — Changes in the DNA of a cell cause it to divide uncontrollably.
2. Tumour formation — The rapidly dividing cells form a mass of abnormal cells (a tumour).
3. Malignant tumours can spread (metastasise) when cells break off and travel through the blood or lymph to other parts of the body, where they form secondary tumours.
Carcinogens — Substances that increase the risk of mutations and cancer. Examples: tobacco smoke, UV radiation, ionising radiation, asbestos, certain chemicals.
Example 3: Benign vs Malignant Tumour
A benign brain tumour is contained within a capsule and grows slowly. It may press on brain tissue and cause symptoms, but it does not spread elsewhere. A malignant lung tumour grows rapidly, breaks through its capsule, and cancer cells can enter the bloodstream and spread to the liver, bones, or brain, forming secondary tumours. The malignant tumour is life-threatening because of its ability to spread and damage multiple organs.
Causal Mechanisms vs Correlation
Correlation — When two variables appear to change together (e.g. as smoking rates increase, lung cancer rates increase). A correlation does NOT prove that one causes the other.
Causal mechanism — A biological explanation that shows HOW one factor directly causes a disease. For example, chemicals in cigarette smoke directly damage DNA in lung cells — this is a causal mechanism, not just a correlation.
Important: A correlation may suggest a link, but it does not prove causation. For example, there may be a correlation between ice cream sales and sunburn rates, but ice cream does not cause sunburn — both are caused by a third factor (sunny weather). To claim a causal link, there must be a known biological mechanism.
Causal mechanism — A biological explanation that shows HOW one factor directly causes a disease. For example, chemicals in cigarette smoke directly damage DNA in lung cells — this is a causal mechanism, not just a correlation.
Important: A correlation may suggest a link, but it does not prove causation. For example, there may be a correlation between ice cream sales and sunburn rates, but ice cream does not cause sunburn — both are caused by a third factor (sunny weather). To claim a causal link, there must be a known biological mechanism.
Example 4: Correlation vs Causation
Studies show a correlation between countries with high fat diets and high rates of breast cancer. However, this correlation alone does not prove that a high-fat diet causes breast cancer — there could be other factors (confounding variables) such as lower physical activity levels, higher alcohol consumption, or genetic differences. To claim a causal link, researchers must identify a biological mechanism explaining HOW dietary fat directly increases breast cancer risk.
Exam tip: When asked about risk factors, always distinguish between correlation and causation. If a causal mechanism is known (e.g. smoking → carcinogens → DNA damage → cancer), you can state that the risk factor causes the disease. If only a correlation is observed, you must say "there is a correlation but this does not prove causation".
Practice Questions
1. Foundation Define the terms communicable disease and non-communicable disease, giving one example of each.
Communicable disease: a disease that can be passed from person to person, caused by pathogens (e.g. measles, caused by a virus). Non-communicable disease: a disease that cannot be passed between people, caused by lifestyle, genetic, or environmental factors (e.g. coronary heart disease, caused by fatty deposits in arteries).
2. Higher Explain how a communicable disease and a non-communicable disease can interact, using a named example.
HIV (communicable) weakens the immune system by destroying white blood cells, making the person more susceptible to other infections such as tuberculosis (another communicable disease) and certain cancers (non-communicable, e.g. Kaposi's sarcoma). The immune deficiency caused by HIV allows these other diseases to develop more easily than they would in a healthy person.
3. Foundation Describe the difference between benign and malignant tumours.
Benign tumours grow slowly, are contained within a capsule, and do not spread to other parts of the body. Malignant tumours grow rapidly, break through their capsule, and can spread to other parts of the body (metastasis) where they form secondary tumours. Malignant tumours are life-threatening; benign tumours are usually not.
4. Higher Explain the difference between correlation and a causal mechanism, using the link between smoking and lung cancer as an example.
A correlation means two variables change together — e.g. higher smoking rates are associated with higher lung cancer rates. A causal mechanism explains HOW one factor directly causes the other — e.g. carcinogens in cigarette smoke damage DNA in lung cells, causing mutations that lead to uncontrolled cell division and cancer. The correlation alone does not prove smoking causes cancer; the causal mechanism (carcinogens damaging DNA) provides the evidence for a direct causal link.
5. Foundation State three risk factors for non-communicable diseases and name a disease linked to each.
1) Smoking — linked to lung cancer and cardiovascular disease. 2) Obesity — linked to Type 2 diabetes. 3) UV exposure — linked to skin cancer. Also acceptable: alcohol (liver disease), lack of exercise (cardiovascular disease).
🔢 Maths Skills
Mathematical Skills
Interpreting epidemiological data: read bar charts, graphs and tables showing disease rates and risk factor data. For example, a graph may show lung cancer incidence per 100,000 people at different smoking levels. Calculate percentage increase: if incidence rises from 20 to 60 per 100,000, the increase = (60 − 20) ÷ 20 × 100 = 200%. Always check axis labels and units carefully.
⚠️ Common Misconceptions
Watch Out!
Students often think risk factors always cause the disease. Wrong: A risk factor guarantees the disease will develop Correct: A risk factor increases the probability of the disease — it is a correlation that may or may not have a proven causal mechanism
Students often think non-communicable diseases can't be prevented. Wrong: Non-communicable diseases cannot be prevented Correct: Lifestyle changes (diet, exercise, not smoking) can significantly reduce the risk of many non-communicable diseases
✍️ 6-Mark Question
Extended Answer
6 marks: Explain the difference between correlation and causation using smoking and lung cancer.
A correlation means two variables change together — for example, populations with higher smoking rates also have higher lung cancer rates. However, correlation alone does not prove that smoking causes lung cancer, because there could be confounding variables (e.g. air pollution, diet, genetics) that affect both. Causation means one factor directly causes the other. For smoking and lung cancer, a causal mechanism is known: carcinogens in cigarette smoke directly damage DNA in lung cells, causing mutations that lead to uncontrolled cell division and tumour formation. This biological mechanism, combined with consistent evidence from large-scale studies that controlled for confounding variables, proves the causal link. It is important to distinguish correlation from causation because not all correlations have a causal mechanism — for example, ice cream sales and sunburn rates correlate but neither causes the other; both are caused by sunny weather.
Mark scheme: 1 mark for defining correlation with a relevant example; 1 mark for defining causation; 1 mark for explaining the causal mechanism (carcinogens → DNA damage → cancer); 1 mark for mentioning confounding variables; 1 mark for explaining why the distinction matters; 1 mark for a clear, structured comparison
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
Evaluate risk factor data: a study shows that in Country A (high smoking rate), lung cancer incidence is 55 per 100,000; in Country B (low smoking rate), it is 12 per 100,000. Country A also has higher air pollution and lower fruit/vegetable consumption. Evaluate whether the data supports the claim that smoking causes lung cancer. The data shows a positive correlation between smoking rate and lung cancer incidence. However, confounding variables (air pollution, diet) have not been controlled, so the correlation alone does not prove causation. The claim is strengthened by the known causal mechanism (carcinogens damaging DNA), but the study would need to control for confounding variables to draw a firm conclusion.
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