# Pathogens and the Spread of Disease

> Types of pathogens, how disease spreads, Koch's postulates and Pasteur's experiments, and diseases of plants and farm animals.

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

### Story time: The doctor who washed his hands
In a Vienna hospital in the 1840s, women giving birth in one ward were dying of "childbed fever" five times more often than in the ward next door. Dr Ignaz Semmelweis noticed the deadly ward was run by doctors who came straight from cutting up dead bodies. The safer ward was run by midwives who did not.

He made the doctors wash their hands in chlorinated water. Deaths fell sharply. But no one yet knew about germs, and many doctors were offended at the idea that their hands were dirty. It took another 20 years, and the work of Louis Pasteur and Robert Koch, before the world accepted that tiny living things cause disease.

**In short:** An infectious disease is caused by a pathogen, such as a virus, bacterium, protist, fungus, parasite or prion, and can spread from one host to another. Pathogens spread by direct and indirect contact, droplets, food and water, and vectors such as mosquitoes.

## Starting point: what you already know

- Microorganisms are living things too small to see without a microscope.
- Bacteria are prokaryotes. Fungi and protists are eukaryotes (see Cell Theory and Microscopes).
- Animals have an immune system that fights infection (Year 9 science).

## Infectious and non-infectious disease

A **disease** is any change that stops the body working normally.

- An **infectious disease** is caused by a **pathogen** (a disease-causing organism or agent) and **can spread** from host to host. Examples: flu, COVID-19, tuberculosis.
- A **non-infectious disease** cannot be passed on. Examples: diabetes, most cancers, cystic fibrosis (Module 8).

## Types of pathogens

| Pathogen | What it is | Example diseases |
|---|---|---|
| **Prions** | Misfolded proteins that make normal proteins misfold too. Not alive | Mad cow disease, Creutzfeldt-Jakob disease |
| **Viruses** | Genetic material in a protein coat. Can only reproduce inside host cells | Flu, COVID-19, measles, HIV, Hendra |
| **Bacteria** | Prokaryotic cells | Tuberculosis, cholera, golden staph, whooping cough |
| **Protists** | Single-celled eukaryotes | Malaria (*Plasmodium*), giardia |
| **Fungi** | Eukaryotes, often thread-like | Tinea (athlete's foot), thrush, chytrid fungus in frogs |
| **Parasites** (macroparasites) | Larger organisms such as worms and arthropods | Tapeworm, head lice, ticks |

Viruses are much smaller than bacteria. Antibiotics kill bacteria but **do not work on viruses**.

## How diseases spread

| Way | Example |
|---|---|
| **Direct contact** (touching, sex, blood) | Herpes, HIV |
| **Indirect contact** (touching contaminated surfaces) | Gastro, golden staph |
| **Airborne droplets** (coughs, sneezes) | Flu, COVID-19, tuberculosis |
| **Contaminated food or water** | Cholera, salmonella |
| **Vectors** (an organism that carries the pathogen) | Mosquitoes spread malaria, dengue and Ross River virus |

Pathogens have **adaptations** for getting into hosts and spreading: flagella to move, sticky proteins to attach to cells, toxins that make the host sneeze or cough out the pathogen.

## Proving germs cause disease

**Louis Pasteur** (1860s):

- His swan neck flask experiment showed microbes come from other microbes, not from broth itself.
- He showed microbes spoiled wine and milk, and invented **pasteurisation** (gentle heating to kill microbes).
- He made early vaccines for chicken cholera, anthrax and rabies.

**Robert Koch** (1880s) found the bacteria that cause anthrax, tuberculosis and cholera. He set out four steps, **Koch's postulates**, to prove a microbe causes a disease.

### Key idea: Koch's postulates
1. The microbe is found in **every** case of the disease, and not in healthy hosts.
2. The microbe is **isolated** and grown in a pure culture.
3. The cultured microbe causes the **same disease** when given to a healthy host.
4. The **same microbe** is isolated again from that new host.

Koch's postulates do not work for every disease. Viruses cannot be grown in pure culture without host cells, and some people carry pathogens without being sick.

## Diseases in plants and farm animals

Disease in crops and livestock affects food supply and the economy.

| Disease | Pathogen | Effect |
|---|---|---|
| Wheat stem rust | Fungus | Destroys wheat crops |
| Panama disease (TR4) | Fungus | Kills banana plants by blocking xylem |
| Myrtle rust | Fungus | Attacks eucalypts, bottlebrushes and paperbarks |
| Foot-and-mouth disease | Virus | Cattle, sheep and pigs. Not in Australia, and strict quarantine keeps it out |
| Ovine Johne's disease | Bacterium | Wasting disease of sheep |

**Mind map**

```text
Pathogens
  Types
    Prions: misfolded proteins
    Viruses: need host cells
    Bacteria: prokaryotes
    Protists: malaria
    Fungi: tinea, rust
    Parasites: worms, ticks
  Spread
    Direct and indirect contact
    Droplets
    Food and water
    Vectors: mosquitoes
  Proof
    Pasteur: flasks, pasteurisation
    Koch's postulates
  Agriculture
    Crop fungi
    Livestock viruses
```

### Year 11: Investigating microbes safely
A classic practical tests whether a common habit spreads microbes, such as touching agar with unwashed and washed fingers. Follow **aseptic technique**:

- Sterilise loops in a flame and work near a Bunsen burner.
- Tape agar plates closed and never open them after incubation.
- Incubate at 25 to 30 degrees, below body temperature.
- Dispose of plates by autoclaving.

Results are usually counted as **colonies**. Each colony grew from one microbe, so counting colonies compares how many microbes were transferred.

**The control.** A sealed agar plate that is never touched shows whether the agar itself was sterile. Any colonies on the test plates can then be blamed on what was added.

**Is a plant disease infectious?** Look for signs that it **spreads**: patches that grow larger, and symptoms that pass to nearby plants. Fungal threads or spores on the leaves are also evidence.

### Year 12: Factors in an epidemic
An **epidemic** is a rapid rise in cases in a region. A **pandemic** spreads across the world (COVID-19 from 2020). The spread depends on

- how the pathogen is **transmitted** and how **infectious** it is,
- how long a person is **contagious** before symptoms show,
- **population density** and **movement** (air travel spreads disease in hours),
- **sanitation** and clean water, and
- how many people are **immune**.

**Zoonoses** are diseases that pass from animals to humans, such as Hendra virus (bats to horses to people) and COVID-19. As people clear forests and move closer to wildlife, zoonoses become more common.

**Identifying a pathogen from what it contains.** Only protein: a **prion**. DNA (or RNA) and protein but no lipid membrane or ribosomes: a **virus**. DNA, RNA, protein, lipid and carbohydrate (a whole cell): a bacterium, protist or fungus.

**Adaptations to survive in a host.** *Helicobacter pylori* lives in the stomach. It makes an enzyme (urease) that produces ammonia, which neutralises the acid around it, and it burrows into the protective mucus layer.

**Where a vector lives decides where a disease spreads.** Dengue is caught in north Queensland because its mosquito, *Aedes aegypti*, lives there. Sydney cases come from travellers, because the mosquito is not established there.

**Threats to farming.** The **varroa mite** feeds on honeybees and spreads viruses, killing hives. Many crops depend on bees for pollination, so yields fall. Foot-and-mouth disease would close Australia's meat and wool export markets and need mass culling, costing billions.

## Quick summary

- Infectious diseases are caused by pathogens and can spread. Non-infectious diseases cannot.
- Pathogens: prions, viruses, bacteria, protists, fungi and parasites.
- Spread: direct and indirect contact, droplets, food and water, and vectors.
- Pasteur showed microbes come from microbes and invented pasteurisation. Koch's postulates prove a microbe causes a disease.
- Plant and animal diseases threaten food supply.

## Quiz

Try these questions from the chapter quiz.

1. A pathogen is
   - A. an organism or agent that causes disease
   - B. a type of white blood cell
   - C. a vaccine
   - D. a non-infectious disease
   - Answer: A. an organism or agent that causes disease. Pathogens cause infectious disease.

2. Which pathogen is not made of cells?
   - A. Bacterium
   - B. Fungus
   - C. Virus
   - D. Protist
   - Answer: C. Virus. Viruses are genetic material in a protein coat.

3. Malaria is caused by a
   - A. virus
   - B. bacterium
   - C. protist
   - D. prion
   - Answer: C. protist. Plasmodium is a single-celled protist.

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