# Causes and Effects of Non-infectious Disease

> Genetic, environmental, nutritional and lifestyle causes of non-infectious disease, cancer, and how epidemiology finds causes and tests treatments.

From HSC Biology on WhizzNotes, free study notes for NSW students: https://whizznotes.com/biology/causes-of-non-infectious-disease/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The doctors who smoked
In 1951, two British scientists, Richard Doll and Austin Bradford Hill, wrote to every doctor in the United Kingdom. They asked a simple question: do you smoke? Then they followed more than 40 000 doctors for the next 50 years, recording who got sick and what they died of.

The answer was clear. Smokers were far more likely to die of lung cancer and heart disease. Doctors who quit lived longer. This study did not use a microscope or a test tube. It used careful counting of large groups of people. That science is called **epidemiology**, and it changed the world's health.

**In short:** Non-infectious diseases cannot be passed from person to person, and are caused by genetics, nutrition, the environment, lifestyle or the body attacking itself. Cancer is uncontrolled cell division caused by mutations, and epidemiology studies how diseases spread through populations.

## Starting point: what you already know

- Infectious diseases are caused by pathogens. Non-infectious diseases are not (see Pathogens and the Spread of Disease).
- Mutations can be caused by radiation and chemicals (see Mutations).
- Homeostasis keeps the body stable, and disease follows when it fails (see Homeostasis).

## Types of non-infectious disease

| Cause | What happens | Examples |
|---|---|---|
| **Genetic** | Inherited faulty alleles or chromosome changes | Cystic fibrosis, haemophilia, Down syndrome, Huntington's disease |
| **Nutritional** | Too little or too much of a nutrient | Scurvy (vitamin C), iron-deficiency anaemia, obesity |
| **Environmental** | Exposure to harmful things | Skin cancer from UV, asbestosis and mesothelioma from asbestos, lung disease from smoke |
| **Lifestyle** | Habits that build up risk over years | Type 2 diabetes, heart disease, many cancers |
| **Autoimmune** | The immune system attacks the body's own cells | Type 1 diabetes, coeliac disease, rheumatoid arthritis |
| **Degenerative** | Body parts wear out or break down with age | Osteoarthritis, macular degeneration, Alzheimer's disease |

Many diseases have **several causes** working together. Heart disease depends on genes, diet, smoking, exercise and age.

## Case studies

**Cystic fibrosis (genetic).** A recessive allele of the CFTR gene makes a faulty chloride channel. Thick mucus clogs the lungs and pancreas, so patients get frequent lung infections and poor digestion. About 1 in 25 Australians is a carrier.

**Melanoma (environmental).** UV light causes mutations in skin cells that control cell division. Australia and New Zealand have the highest melanoma rates in the world. Early detection by skin checks greatly improves survival.

**Type 2 diabetes (lifestyle and genetic).** Cells stop responding properly to insulin, so blood glucose stays high. Risk rises with obesity, inactivity and family history. It can damage eyes, kidneys, nerves and blood vessels. It is often managed with diet, exercise and medicine.

## Cancer

**Cancer** is uncontrolled cell division. Normally, genes tightly control when a cell divides. Mutations in two types of genes can cause cancer:

- **Proto-oncogenes** tell cells to divide. A mutation can jam them "on", turning them into **oncogenes**.
- **Tumour suppressor genes** (like *p53*) stop division or repair DNA. A mutation can switch them "off".

Usually several mutations are needed. A mass of dividing cells is a **tumour**. **Benign** tumours stay in one place. **Malignant** tumours invade nearby tissue and can spread through the blood or lymph to other parts of the body (**metastasis**).

## Epidemiology

**Epidemiology** is the study of the patterns, causes and effects of disease in populations.

| Type of study | What it does | Example |
|---|---|---|
| **Descriptive** | Describes who gets a disease, where and when | Mapping melanoma rates in each state |
| **Analytical** (cohort or case-control) | Compares groups to find causes | The British doctors study comparing smokers and non-smokers |
| **Intervention** (clinical trial) | Tests a treatment or prevention by giving it to one group and not another | Testing a new vaccine or drug |

### Key idea: Measuring disease
- **Incidence**: the number of **new** cases in a period, often per 100 000 people per year.
- **Prevalence**: the **total** number of people who have the disease at one time.
- **Mortality**: the number of **deaths** from the disease.

### Worked example: Comparing incidence
In one year, a town of 50 000 people has 30 new cases of melanoma. Incidence per 100 000:
$$\frac{30}{50\,000} \times 100\,000 = 60 \text{ per } 100\,000$$

**Mind map**

```text
Non-infectious disease
  Causes
    Genetic: cystic fibrosis
    Nutritional: scurvy
    Environmental: UV, asbestos
    Lifestyle: type 2 diabetes
    Autoimmune: type 1 diabetes
  Cancer
    Uncontrolled division
    Oncogenes on
    Tumour suppressors off
    Benign or malignant
    Metastasis
  Epidemiology
    Descriptive
    Analytical
    Intervention trials
    Incidence, prevalence, mortality
```

### Year 11: Designing and judging a clinical trial
A good **intervention study** (randomised controlled trial):

- has a **large sample** of people,
- assigns people **randomly** to a treatment group and a **control** group,
- gives the control group a **placebo** (a fake treatment that looks the same),
- is **double blind**: neither the patients nor the researchers know who gets the real treatment, which avoids bias, and
- is approved by an **ethics committee**, with **informed consent** from everyone.

Epidemiology can show a strong **link** (correlation) between a factor and a disease. To argue cause, scientists look for a **dose-response** pattern (more smoking, more risk), a link that appears in many studies, and a biological reason (chemicals in smoke damage DNA).

**Cohort or case-control?** A **cohort** study follows a group forward in time and records who gets the disease (like the British doctors). A **case-control** study starts with people who already have the disease and compares their past with similar people who do not.

**More reliable results** come from a larger sample, and from repeating the study in other groups.

**Correlation is not cause.** Finding that people who eat more processed meat get more bowel cancer shows a link, but other factors (like smoking or exercise) might explain it.

### Year 12: Prevention programs in Australia
Epidemiology guides health programs. HSC questions often ask you to evaluate one.

- **Cancer screening**: the National Bowel Cancer Screening Program mails free test kits to people aged 45 to 74. BreastScreen and the cervical screening program (testing for HPV) catch disease early.
- **HPV vaccination**: Australian scientists Ian Frazer and Jian Zhou developed the technology behind it. Australia is on track to eliminate cervical cancer as a public health problem by about 2035.
- **SunSmart**: "Slip, slop, slap" since 1981, with UV alerts and shade in schools. Melanoma rates in younger Australians have started to fall.
- **Newborn screening**: a heel-prick blood test checks every baby for conditions like cystic fibrosis.

A good evaluation looks at **cost**, **reach**, **participation rates**, **evidence of reduced incidence or mortality**, and **equity** for groups such as rural and Aboriginal and Torres Strait Islander communities.

**More rates.** Prevalence: 1200 people with diabetes in a town of 40 000 is $\dfrac{1200}{40\,000} \times 100 = 3\%$. Mortality: 150 deaths in 2 500 000 people is $\dfrac{150}{2\,500\,000} \times 100\,000 = 6$ per 100 000. Using "per 100 000" lets places of different sizes be compared fairly.

**Relative risk** $= \dfrac{\text{rate in exposed group}}{\text{rate in unexposed group}}$. Smokers 140 and non-smokers 10 per 100 000 gives a relative risk of 14: smokers are 14 times as likely to die of lung cancer.

**Long delays.** Cancers take decades to develop after exposure, so mesothelioma cases kept rising long after asbestos was banned, and lung cancer deaths fell about 20 years after smoking rates fell.

**Mother's age and Down syndrome.** Eggs are formed before birth and wait for decades. Older eggs are more likely to have non-disjunction in meiosis.

**Tricky categories.** Skin cancer is non-infectious because it cannot pass between people, even though the environment causes it. Cervical cancer is usually started by HPV, an infectious virus, but the cancer itself cannot spread to another person; vaccination against HPV prevents it.

## Quick summary

- Non-infectious diseases can be genetic, nutritional, environmental, lifestyle-related, autoimmune or degenerative. Many have several causes.
- Cancer is uncontrolled cell division caused by mutations in oncogenes and tumour suppressor genes.
- Malignant tumours can spread by metastasis.
- Epidemiology studies disease in populations: descriptive, analytical and intervention studies.
- Incidence counts new cases. Prevalence counts all current cases.

## Quiz

Try these questions from the chapter quiz.

1. Which is a genetic disease?
   - A. Cystic fibrosis
   - B. Scurvy
   - C. Influenza
   - D. Mesothelioma
   - Answer: A. Cystic fibrosis. It is caused by an inherited faulty allele.

2. Scurvy is caused by a lack of
   - A. iron
   - B. vitamin D
   - C. vitamin C
   - D. protein
   - Answer: C. vitamin C. It is a nutritional disease.

3. Which is an environmental cause of disease?
   - A. Inheriting a faulty allele
   - B. Exposure to asbestos
   - C. An extra chromosome
   - D. Getting older
   - Answer: B. Exposure to asbestos. Asbestos causes lung disease and cancer.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/biology/causes-of-non-infectious-disease/#quiz