an organism or agent that causes disease

Pathogens and the Spread of 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.

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

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 11Investigating 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 12Factors 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

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Try 3 questions now

A pathogen is
1. A an organism or agent that causes disease
2. B a type of white blood cell
3. C a vaccine
4. D a non-infectious disease
Show answer
A: an organism or agent that causes disease
Pathogens cause infectious disease.
Which pathogen is not made of cells?
1. A Bacterium
2. B Fungus
3. C Virus
4. D Protist
Show answer
C: Virus
Viruses are genetic material in a protein coat.
Malaria is caused by a
1. A virus
2. B bacterium
3. C protist
4. D prion
Show answer
C: protist
Plasmodium is a single-celled protist.