Describe a variety of infectious diseases caused by pathogens, including microorganisms, macroorganisms and non-cellular pathogens, and collect primary and secondary-sourced data and information relating to disease transmission, including: classifying different pathogens that cause disease in plants and animals

What this dot point is asking

NESA wants you to classify the six main types of pathogen, describe a defining structural feature of each, and give at least one named example of a disease they cause in plants or animals. This is foundational content that appears in multiple choice every year and underpins almost every Module 7 extended response.

The answer

A pathogen is any biological agent that causes disease in a host. Pathogens fall into six categories: prions, viruses, bacteria, protozoa, fungi and macroparasites. The diagram below shows their typical size range, which spans six orders of magnitude.

Prions

Structure

Misfolded proteins. No nucleic acid, no cell structure.

Mechanism

A prion induces normal cellular proteins (often PrP in nervous tissue) to misfold into the same abnormal shape, creating aggregates that destroy brain tissue.

Example

Bovine spongiform encephalopathy (BSE, "mad cow disease") and the human variant Creutzfeldt-Jakob disease.

Viruses

Structure

Acellular particles. Nucleic acid (DNA or RNA) enclosed in a protein capsid, sometimes with a lipid envelope. Not considered living.

Mechanism

Cannot replicate independently. Inject genetic material into a host cell and hijack the host's machinery to produce new viral particles.

Examples

Influenza A (RNA virus, respiratory), HIV (retrovirus, immune cells), tobacco mosaic virus (plant pathogen affecting tomato and tobacco leaves).

Bacteria

Structure

Prokaryotic single-celled organisms. Cell wall (peptidoglycan), plasma membrane, cytoplasm, 70S ribosomes, circular DNA, often with plasmids. No nucleus.

Mechanism

Cause disease by producing toxins (e.g. Clostridium tetani releases tetanospasmin) or by colonising and damaging host tissue.

Examples

Mycobacterium tuberculosis (tuberculosis), Vibrio cholerae (cholera), Agrobacterium tumefaciens (crown gall in plants).

Protozoa

Structure

Single-celled eukaryotes. Have a nucleus, membrane-bound organelles and often complex life cycles.

Mechanism

Often transmitted by vectors. Invade specific tissues and reproduce inside host cells.

Examples

Plasmodium falciparum (malaria, transmitted by Anopheles mosquito), Trypanosoma brucei (African sleeping sickness, tsetse fly).

Fungi

Structure

Eukaryotic, either unicellular (yeasts) or multicellular (moulds with hyphae). Cell walls made of chitin.

Mechanism

Often opportunistic, infecting compromised tissue or hosts. Spread by spores.

Examples

Tinea pedis (athlete's foot in humans), Candida albicans (thrush), Puccinia graminis (wheat stem rust, a major plant pathogen).

Macroparasites

Structure

Multicellular eukaryotic organisms, often with complex life cycles. Includes helminths (worms) and ectoparasites (fleas, ticks).

Mechanism

Live in or on the host, drawing nutrients and causing tissue damage, blood loss or immune dysfunction.

Examples

Taenia solium (pork tapeworm), Schistosoma mansoni (blood fluke causing schistosomiasis), Phytophthora infestans (a protist-like macroparasite causing potato blight).

Examples in context

Example 1. Phytophthora cinnamomi causing dieback in Western Australian jarrah forests. Phytophthora cinnamomi is a water mould (oomycete), neither true fungus nor bacterium, that causes dieback in over 40 percent of jarrah (Eucalyptus marginata) forests in southwestern Australia. The pathogen reproduces by motile zoospores that swim through soil water to root tips, where they encyst and germinate, then their hyphae invade and destroy the xylem so the tree cannot transport water. Parks and Wildlife Service WA classifies dieback as the single greatest threat to native flora biodiversity. Quarantine measures including boot-washing stations and vehicle hygiene at park entrances aim to limit pathogen spread, which illustrates how recognising the pathogen type (a water-borne stramenopile) directly shapes control strategy.

Example 2. Hendra virus from flying foxes to horses in NSW. Hendra virus is a paramyxovirus (RNA virus, enveloped) that naturally infects Australian black and grey-headed flying foxes (Pteropus species) without causing disease. Periodically, viral particles in flying fox urine or partially eaten fruit contaminate horse paddocks, infecting horses through ingestion or mucous membrane contact. Affected horses develop fatal respiratory and neurological disease within days. Since 1994, 80 horses and four humans have died in NSW and Queensland outbreaks. Because Hendra is an RNA virus with a lipid envelope, antiviral options are limited; instead control depends on a horse vaccine (Equivac HeV, available since 2012) and removing horses' access to flying fox roosting trees.

Try this

Q1. Classify the following pathogens by type: (a) Bacillus anthracis, (b) Plasmodium falciparum, (c) HIV, (d) Cordyceps fungus. [4 marks]

• Cue. (a) Bacterium (prokaryote). (b) Protozoan (eukaryotic single cell). (c) Virus (RNA retrovirus). (d) Fungus (eukaryotic, hyphal).

Q2. A new disease outbreak in NSW poultry shows symptoms within 48 hours. Electron microscopy reveals 80 nm enveloped particles with a single-stranded RNA genome. Identify the most likely pathogen type and explain two control strategies. [3 marks]

• Cue. Likely a virus (e.g. avian influenza). Control: culling infected flocks, vaccination of healthy birds, biosecurity quarantine zones.

Q3. Compare bacteria and viruses. (a) Describe one structural difference. (b) Describe one reproductive difference. (c) Explain why antibiotics work against bacteria but not viruses. [2+2+2 marks]

• Cue. (a) Bacteria are cellular with cell walls; viruses are acellular with protein capsids. (b) Bacteria binary fission; viruses require host cell machinery. (c) Antibiotics target bacterial-specific structures (e.g. peptidoglycan) absent in viruses.