Describe the main groups of pathogens (bacteria, viruses, fungi, protists, prions) and their modes of transmission, distinguishing between communicable and non-communicable disease

What this dot point is asking

QCAA expects you to name the five main groups of pathogens, give a structural feature and an example human disease for each, and identify the main modes of disease transmission. You should also distinguish communicable (infectious) from non-communicable disease.

The answer

A pathogen is an organism (or non-living agent in the case of prions and viruses) that causes disease in a host. The five groups assessed at QCE level are bacteria, viruses, fungi, protists and prions.

The five groups of pathogens

Bacteria. Prokaryotic, single-celled organisms with circular DNA, 70S ribosomes and (in most species) a peptidoglycan cell wall. Reproduce by binary fission. Cause disease either by damaging tissues directly or by producing toxins.

• Mycobacterium tuberculosis (tuberculosis): respiratory.
• Streptococcus pneumoniae (pneumonia): respiratory.
• Salmonella species (gastroenteritis): foodborne.
• Vibrio cholerae (cholera): waterborne; cholera toxin disrupts ion balance in the gut.
• Neisseria gonorrhoeae (gonorrhoea): sexually transmitted.

Viruses. Non-cellular: nucleic acid (DNA or RNA) inside a protein capsid, sometimes with a lipid envelope studded with glycoproteins. Cannot reproduce on their own; they hijack host cell machinery to replicate.

• Influenza A virus (influenza): respiratory droplet.
• SARS-CoV-2 (COVID-19): respiratory droplet and aerosol.
• Human immunodeficiency virus (HIV) (AIDS): blood-borne, sexual, vertical.
• Hepatitis B and C viruses: blood-borne.
• Varicella-zoster virus (chickenpox and shingles): respiratory and contact.

Fungi. Eukaryotic; chitin cell walls. Mostly multicellular networks of hyphae; some are single-celled yeasts. Cause disease through tissue invasion, allergic reactions or mycotoxins.

• Tinea species (ringworm, athlete's foot): skin contact.
• Candida albicans (thrush): opportunistic; overgrowth on mucous membranes.
• Aspergillus fumigatus (aspergillosis): airborne spores; lung infection in immunocompromised people.

Protists. Eukaryotic, mostly unicellular. Varied structures including flagella, cilia or pseudopodia. Many have complex life cycles involving vectors.

• Plasmodium species (malaria): vector-borne via Anopheles mosquitoes.
• Trypanosoma species (sleeping sickness, Chagas disease): vector-borne via tsetse flies and triatomine bugs.
• Giardia intestinalis (giardiasis): waterborne.
• Toxoplasma gondii (toxoplasmosis): foodborne or via cat faeces.

Prions. Misfolded versions of a normal cellular protein (PrP). They convert correctly folded copies into the misfolded form, producing aggregates that damage neurons. No nucleic acid is involved.

• Creutzfeldt-Jakob disease (CJD) and variant CJD in humans.
• Bovine spongiform encephalopathy (BSE, "mad cow disease") in cattle.
• Scrapie in sheep.

Prions are resistant to standard sterilisation (heat, UV, formaldehyde) because they have no nucleic acid to disrupt.

Communicable vs non-communicable disease

• Communicable (infectious). Caused by a pathogen and transmissible from person to person, animal to person or environment to person. All five pathogen groups produce communicable diseases.
• Non-communicable. Not caused by a transmissible pathogen. Examples: cardiovascular disease, type 2 diabetes, most cancers, asthma, autoimmune disorders.

Some diseases sit at the boundary: certain cancers (cervical cancer, liver cancer) are triggered by communicable infections (HPV, Hepatitis B and C) but the cancers themselves are not transmissible.

Modes of transmission

Most communicable diseases use one or more of the following routes.

• Direct contact. Touching the infected person (impetigo), sexual contact (chlamydia, gonorrhoea, HIV), or contact with infected fluids.
• Droplet (respiratory droplets). Large droplets travel less than 2 metres after a cough or sneeze; whooping cough, COVID-19, influenza.
• Airborne (aerosols). Smaller particles can remain suspended for longer and travel further; tuberculosis, measles.
• Vector-borne. An arthropod or other organism carries the pathogen between hosts; mosquitoes carry malaria, dengue, Ross River virus and Zika; ticks carry Lyme disease.
• Waterborne and foodborne (faecal-oral). Pathogens shed in faeces enter via contaminated water or food; cholera, typhoid, hepatitis A, E. coli enteritis, Salmonella.
• Blood-borne. Pathogens in blood transmitted by shared needles, transfusion or sexual contact; HIV, Hepatitis B and C.
• Vertical. From mother to fetus during pregnancy, birth or breastfeeding; rubella, HIV, syphilis.
• Fomite. Inanimate object (door handle, towel) carrying viable pathogen from one host to another. Common for norovirus and rhinoviruses.

Australian context

• Ross River virus. Mosquito-borne; widespread in regional Queensland.
• Hendra virus. Bat to horse to human; rare but serious; first identified in Brisbane.
• Q fever. Coxiella burnetii from livestock to humans; airborne dust, occupational hazard for abattoir workers and farmers.
• Influenza. Annual outbreaks; vaccines updated each year.

Common traps

Calling viruses bacteria. Viruses are non-cellular and far smaller. Antibiotics that target bacteria (cell wall, ribosomes) do not work on viruses.

Forgetting prions. Prions are the smallest assessable pathogen group at QCE level; expect them to come up specifically because they are unusual.

Treating all transmission as droplet. Mode matters for control. Waterborne pathogens need clean water, vector-borne pathogens need vector control, blood-borne pathogens need safe injecting and screened blood, droplet pathogens need distance and ventilation.

Mixing up communicable and contagious. Communicable diseases include all transmissible infections, including vector-borne and foodborne ones; contagious is usually reserved for highly transmissible person-to-person diseases.

Cross-link to Year 12 assessment

This dot point sets up innate and adaptive immunity (what the body does about pathogens) and vaccines and antibiotic resistance. Bacterial and viral case studies often appear in Unit 4 IA3 research investigations on biotechnology (antibiotic discovery, vaccine design) and in EA short-response questions on the immune response.

In one sentence

Pathogens are organisms (bacteria, fungi, protists) or non-living agents (viruses, prions) that cause disease, distinguished by their structural features (prokaryotic cells, capsids, chitin walls, eukaryotic single cells, misfolded proteins) and transmitted by direct contact, droplet, airborne, vector, faecal-oral, blood-borne or vertical routes to produce communicable disease, in contrast to non-communicable disease which has no transmissible cause.