AQA GCSE Biology: Health and Non-Communicable Disease

Health is about more than not being ill, and many diseases are linked to how we live. This guide explains what health means, how different diseases can interact, the main risk factors for non-communicable diseases, and the important difference between correlation and causation. That last idea is one examiners return to again and again.

What health means

Health is a state of physical and mental well-being, not just the absence of disease.

Ill health can have several causes: communicable diseases (caused by pathogens, and able to spread between organisms), non-communicable diseases (not caused by pathogens, and not spread), and factors such as poor diet, stress and life circumstances (for example financial hardship or poor living conditions).

How diseases interact

Different diseases and conditions can influence one another.

• A weakened immune system makes a person more likely to catch infectious diseases.
• Some viruses can trigger cancers by living inside cells.
• Immune reactions first caused by a pathogen can trigger allergies, such as asthma or skin rashes.
• Severe physical illness can lead to depression and other mental health problems.

Risk factors

A risk factor is an aspect of a person's lifestyle, or a substance in their body or environment, that is linked to an increased rate of a disease. Many non-communicable diseases result from several risk factors interacting.

• Obesity is a major risk factor for type 2 diabetes.
• A diet high in saturated fat raises blood cholesterol, speeding up the build-up of fatty deposits in arteries.
• Smoking damages the lining of arteries and raises blood pressure.
• Lack of exercise raises blood pressure.
• Alcohol damages the liver (its toxic breakdown products can cause scarring, or cirrhosis, leading to liver failure) and the brain, and increases the risk of liver cancer.
• Smoking damages the lungs: it harms the alveoli, reducing the surface area for gas exchange; it inflames the bronchi and damages the cilia that clear mucus; and the tar contains carcinogens that increase the risk of lung cancer.

Some risk factors affect unborn babies. Carbon monoxide in cigarette smoke binds to the mother's haemoglobin, reducing its oxygen-carrying capacity, so the foetus gets less oxygen, raising the risk of premature birth, low birth weight or stillbirth. Alcohol in pregnancy increases the risk of miscarriage, premature birth and foetal alcohol syndrome.

Correlation and causation

It is vital to tell these two apart.

• A correlation means that as one factor changes, another changes too, so they are linked.
• Causation means one factor directly causes the other.

A correlation on its own does not prove causation, because a third, confounding, factor might explain the link. Causation is only established by further evidence that shows a clear mechanism.

Smoking and lung cancer is the example to quote, because it shows what the extra evidence looks like. The correlation was noticed first, and on its own it could have been explained away: perhaps people who smoked also tended to live in more polluted places. It became accepted as causation when a mechanism was identified, namely that the tar in cigarette smoke contains carcinogens that cause mutations in lung cells, supported by evidence from animal studies and from the risk falling in people who stopped smoking.

When a question gives you data and asks whether one thing causes another, the safe answer names the correlation, says a correlation alone cannot prove cause, suggests a plausible confounding factor, and states what further evidence would be needed.

The cost of non-communicable disease

Non-communicable diseases carry both a human and a financial cost, and AQA asks about them at four scales.

• To the individual: a shorter life, long-term ill health, and the cost of treatment or of being unable to work.
• To the family: lost income, and the burden of caring for someone who is ill.
• To the nation: the cost to the health service of treating millions of long-term conditions, plus the economic cost of people being too unwell to work.
• Globally: non-communicable diseases are now a leading cause of death worldwide, including in poorer countries, which strains health systems that are already limited.

This is why so much public health effort goes into the risk factors rather than the treatments. Reducing smoking, obesity and alcohol consumption across a whole population prevents disease far more cheaply than treating it afterwards.

How this fits together

Cancer is one of the most important non-communicable diseases, and it has its own set of causes and features, covered in the cancer guide.