What you need to know
- Distinguish communicable and non-communicable disease.
- Describe health as physical, mental and social wellbeing.
- Explain how several risk factors can interact.
- Distinguish correlation from causation and identify confounding variables.
- Evaluate health evidence, media claims and population data.
Health and disease
Health includes physical, mental and social wellbeing, not simply the absence of diagnosed illness. A communicable disease is caused by a pathogen and can be transmitted directly or indirectly. A non-communicable disease is not caused by a transmissible pathogen and cannot be passed between people.
Examples include cardiovascular disease, many cancers, type 2 diabetes and chronic liver disease. Some persist for years and create major personal, social and economic costs.
Risk factors and mechanisms
| Risk factor | Associated outcomes | Possible biological link |
|---|---|---|
| Smoking | Lung disease, cardiovascular disease and several cancers. | Carcinogens damage DNA; carbon monoxide reduces oxygen transport; smoke damages lungs and arteries. |
| Alcohol excess | Liver and brain damage and increased risk of some cancers. | Alcohol and its breakdown products damage cells; long-term exposure can cause inflammation and scarring. |
| Energy-dense diet and inactivity | Obesity, type 2 diabetes and cardiovascular disease. | Long-term energy imbalance and altered glucose and lipid control. |
| Ionising radiation | Increased cancer risk. | Radiation can damage DNA and cause mutations. |
| Inherited variants | Different susceptibility to many diseases. | Alleles can affect cell control, metabolism or blood lipids. |
Risk factors alter probability. Dose, duration, age, inherited susceptibility and combinations of exposures all matter.
Diseases can interact
- One disease can weaken defences and increase susceptibility to another.
- Some infections increase the risk of later cancer.
- Obesity can raise the risk of type 2 diabetes and cardiovascular disease.
- Physical illness can affect mental and social wellbeing, while mental ill health can influence health behaviours and treatment.
Because several factors act together, a simple single-cause explanation is often incomplete.
Correlation is not automatically causation
Correlation
Two measured variables change together. This is a pattern in data.
Causation
A change in one factor produces a change in the other through a supported mechanism.
A correlation may arise because A causes B, B causes A, a third confounding variable affects both, the sample is biased, or the pattern occurs by chance.
Evidence for causation becomes stronger when findings are repeated in large representative studies, exposure occurs before the outcome, higher exposure is linked with greater effect, plausible mechanisms exist and alternative explanations are controlled.
Misconception AlertIf there is no absolute proof, there is no scientific evidence.Select to reveal the correctionSelect to hide the correction
Scientific conclusions are based on the strength, consistency and limitations of evidence. Evidence can be strong without providing absolute proof.
Local, national and global patterns
Disease rates differ with age structure, income, occupation, environment, access to healthcare, reporting quality, genetics and behaviour. A raw number of cases is affected by population size, so rates per 1000 or 100 000 people are often more useful.
Differences between countries do not identify one cause because many variables differ at the same time.
Evaluating health and media claims
- Check sample size, selection and whether a control or comparison group was used.
- Look for absolute risk as well as relative risk.
- Check whether the result is statistically convincing and independently repeated.
- Identify funding, conflicts of interest and whether the report links to the original study.
- Separate an observational association from a controlled experiment.
- Ask whether the conclusion applies to the population studied.
If risk rises from 1 in 10 000 to 2 in 10 000, it has doubled in relative terms, but the absolute increase is 1 additional case per 10 000.
Quick retrieval check
1. What is a non-communicable disease?
2. What is a confounding variable?
3. Why are disease rates more useful than case numbers when comparing countries?
4. Give two features that strengthen evidence for causation.
Exam connection
A study finds that people who exercise more have a lower rate of heart disease. Explain why this result alone does not prove that exercise caused the difference.
Show the mark points
- The result shows correlation.
- Groups may also differ in diet, smoking, age, income or healthcare.
- These confounding variables could affect heart-disease risk.
- Further controlled, repeated evidence and a plausible mechanism are needed.