# Preventing, Treating and Controlling Infectious Disease

> Vaccination and herd immunity, hygiene and quarantine, antibiotics and antiviral drugs, antibiotic resistance, Aboriginal bush medicine and global disease control.

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

### Story time: Sniffer dogs at the airport
Every visitor arriving in Australia fills in a card asking about food, plants and animals. Sniffer dogs walk through airport halls. It can seem fussy. But Australia has no rabies, no foot-and-mouth disease and fewer crop diseases than most countries, partly because of this strict **biosecurity**.

In 2001 an outbreak of foot-and-mouth disease in the United Kingdom led to over six million sheep, cows and pigs being culled and cost billions of pounds. One study estimated a large outbreak in Australia could cost around 80 billion dollars. Keeping a disease out is far cheaper than fighting it once it arrives.

**In short:** Epidemics are prevented by stopping a disease from spreading, with hygiene, quarantine, vector control and contact tracing, and by vaccination, which gives immunity without illness. Antibiotics treat only bacterial infections, and overusing them makes bacteria resistant.

## Starting point: what you already know

- Pathogens spread by contact, droplets, food and water, and vectors (see Pathogens and the Spread of Disease).
- Memory cells give long-term immunity (see The Immune Response).
- Antibiotics kill bacteria but not viruses.

## Stopping the spread

| Strategy | How it works | Example |
|---|---|---|
| **Hygiene** | Removes or kills pathogens before they reach a host | Hand washing, covering coughs, cooking food well |
| **Sanitation** | Clean water and safe sewage disposal | Prevents cholera and typhoid |
| **Quarantine** | Keeping people, animals or goods separate until they are shown to be disease-free | Biosecurity at borders. COVID-19 hotel quarantine |
| **Isolation** | Separating sick people | Hospital isolation wards |
| **Vector control** | Reducing the animals that carry pathogens | Removing standing water where mosquitoes breed. Insect repellent |
| **Contact tracing** | Finding and testing people who were near a case | Used for measles, tuberculosis and COVID-19 |

## Vaccination

A **vaccine** contains antigens from a pathogen in a harmless form: killed or weakened pathogens, pieces of the pathogen, an inactivated toxin, or mRNA instructions to make one pathogen protein. The body makes antibodies and **memory cells** without the person getting sick.

**Herd immunity**: when enough people are immune, the pathogen cannot find new hosts and stops spreading. This protects people who cannot be vaccinated, such as newborn babies and people having chemotherapy. Measles is very infectious, so about 95% of people need to be immune.

Smallpox was declared **eradicated** in 1980 after a worldwide vaccination program. It is the only human disease ever wiped out.

## Treating disease

**Antibiotics** kill bacteria or stop them growing, for example by damaging their cell walls (penicillin) or their ribosomes. They do **not** work on viruses, because viruses have no cell wall or ribosomes of their own.

**Antivirals** slow viruses from copying themselves inside cells, for example drugs for HIV, influenza and COVID-19.

**Antifungals** treat fungal infections like tinea and thrush.

### Watch out: Antibiotics for a cold
A cold is caused by a virus. Antibiotics will not help, and using them when they are not needed speeds up **antibiotic resistance**.

## Antibiotic resistance

When antibiotics are overused or a course is not finished, bacteria with resistance mutations survive and multiply (natural selection). Some bacteria, like **MRSA** (golden staph resistant to many antibiotics), are now very hard to treat. To slow resistance:

- only use antibiotics for bacterial infections,
- take the whole course as prescribed, and
- reduce their use in farm animals.

## Aboriginal and Torres Strait Islander knowledge of medicine

Aboriginal and Torres Strait Islander peoples have used many plants for healing for thousands of years. For example, the leaves of the **smoke bush** and **emu bush** were used by some groups to treat wounds and infections. Scientists have since found antibacterial chemicals in many of these plants. **Tea tree oil**, used by the Bundjalung people of northern New South Wales, is now known to kill many bacteria and fungi.

This knowledge belongs to the communities who hold it. Under **ICIP** principles and benefit-sharing laws, research must have the community's consent, credit the knowledge holders and share any benefits fairly. Using traditional knowledge without permission to make a commercial product is called **biopiracy**.

**Mind map**

```text
Controlling disease
  Prevent spread
    Hygiene and sanitation
    Quarantine and isolation
    Vector control
    Contact tracing
  Vaccination
    Memory without illness
    Herd immunity
    Smallpox eradicated
  Treatment
    Antibiotics: bacteria only
    Antivirals
    Antifungals
  Resistance
    Natural selection
    Finish the course
  Bush medicine
    Tea tree oil
    ICIP and consent
```

### Year 11: Evaluating strategies for an epidemic
When an exam asks you to evaluate methods of controlling a disease, link each method to how the **pathogen spreads**.

- **Dengue** and **malaria** (vector-borne): mosquito control, nets and repellent work best. The **Wolbachia** program released mosquitoes carrying a bacterium that stops them passing on dengue. Dengue cases in Townsville fell to almost zero.
- **Cholera** (water-borne): clean water and sanitation matter most.
- **Measles** (airborne, very infectious): high vaccination rates are essential.

Consider **cost**, **effectiveness**, **side effects**, and whether people will **follow** the measure.

**How long to quarantine?** Quarantine must last longer than the disease's **incubation period** (the time from infection to symptoms), so anyone infected will show it.

**Ebola (2014).** It spreads through body fluids, so isolation of patients, protective equipment for health workers, contact tracing and safe burials were the main controls.

**Testing antibiotics.** Paper discs soaked in different antibiotics are placed on a plate of bacteria. The bigger the clear zone around a disc, the better that antibiotic stops the bacteria. A zone of 0 mm means the bacteria are resistant to it.

### Year 12: Global and local factors in an outbreak
Controlling disease needs action at many levels:

- **Local**: health departments trace contacts and run vaccination clinics.
- **National**: the government controls borders and funds research. Australia's National Immunisation Program gives free vaccines.
- **Global**: the **World Health Organization** (WHO) tracks outbreaks and coordinates responses, as with COVID-19 and Ebola.

Factors that make outbreaks worse include poverty, crowding, lack of clean water, war, climate change (spreading mosquitoes to new areas) and mistrust of health advice. Successful programs work **with** communities. Aboriginal Community Controlled Health Organisations led some of Australia's most effective COVID-19 vaccination efforts in remote communities.

**Herd immunity from the reproduction number.** If each case infects $R_0$ others, the fraction that must be immune is $1 - \dfrac{1}{R_0}$. For $R_0 = 4$, that is $1 - \dfrac{1}{4} = \dfrac{3}{4}$, or 75%.

**Case fatality rate** $= \dfrac{\text{deaths}}{\text{cases}} \times 100$. 3000 deaths from 12 000 cases is 25%.

**Vaccine details.**
- **Live attenuated** vaccines contain weakened living pathogens. In someone with a weak immune system they could multiply and cause disease, so they are not given.
- **Boosters**: several doses months apart give a secondary response each time, making more memory cells and longer protection.
- If a disease comes back decades after a vaccine, the likely cause is fewer people being vaccinated (or immunity fading), so herd immunity is lost.

## Quick summary

- Hygiene, sanitation, quarantine, isolation, vector control and contact tracing stop the spread of disease.
- Vaccines give memory cells without illness. Herd immunity protects those who cannot be vaccinated.
- Antibiotics treat bacterial infections only. Antivirals treat viral infections.
- Overuse of antibiotics leads to resistant bacteria through natural selection.
- Aboriginal bush medicine knowledge is valuable and is protected by ICIP: research needs consent and benefit sharing.

## Quiz

Try these questions from the chapter quiz.

1. A vaccine works by
   - A. killing all bacteria in the body
   - B. causing the body to make memory cells without causing the disease
   - C. giving ready-made antibodies only
   - D. curing a disease already present
   - Answer: B. causing the body to make memory cells without causing the disease. Vaccines train the immune system.

2. Herd immunity means
   - A. enough people are immune that the pathogen cannot spread well
   - B. all animals in a herd are vaccinated
   - C. one person protects their family
   - D. the disease disappears in one day
   - Answer: A. enough people are immune that the pathogen cannot spread well. Few hosts are left for the pathogen.

3. Antibiotics should be used for
   - A. colds and flu
   - B. bacterial infections
   - C. all viral infections
   - D. every fever
   - Answer: B. bacterial infections. They do not work on viruses.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/biology/preventing-epidemics/#quiz