Classify pathogens and explain how infectious diseases are transmitted

What this dot point is asking

SCSA wants you to classify pathogens, distinguish infectious from non-infectious disease, and explain the modes of transmission. A strong answer matches each pathogen type to examples and links transmission routes to how diseases spread through populations.

Infectious versus non-infectious disease

A disease is any condition that impairs normal functioning of an organism. Diseases fall into two broad groups:

• Infectious (communicable) diseases are caused by pathogens and can be passed from one organism to another (for example, influenza, tuberculosis, malaria).
• Non-infectious (non-communicable) diseases are not caused by transmissible pathogens and cannot be caught (for example, genetic disorders, cancers, nutritional deficiencies, cardiovascular disease).

This dot point focuses on infectious disease and its agents.

Types of pathogen

A pathogen is any agent that causes disease. The main groups are:

• Bacteria: single-celled prokaryotes. Some cause disease by releasing toxins or damaging tissue (for example, tuberculosis, tetanus, salmonella). Treatable with antibiotics.
• Viruses: non-cellular agents of nucleic acid in a protein coat. They are obligate intracellular parasites that hijack host cells to replicate, destroying them (for example, influenza, HIV, measles). Not affected by antibiotics.
• Fungi: eukaryotic organisms such as yeasts and moulds, causing diseases like tinea (athlete's foot) and thrush.
• Protozoa: single-celled eukaryotes, often spread by vectors (for example, Plasmodium, which causes malaria).
• Prions: misfolded proteins that cause other proteins to misfold, damaging nervous tissue (for example, the agent of mad cow disease). They contain no nucleic acid.

Larger parasites such as worms (helminths) can also cause disease.

How pathogens cause harm

Pathogens cause disease (their pathogenicity) in several ways: by directly destroying host cells as they reproduce (many viruses), by releasing toxins that disrupt body functions (many bacteria), by competing for nutrients, or by triggering damaging immune responses. The severity often depends on the dose of pathogen and the host's resistance.

Modes of transmission

For an infectious disease to spread, the pathogen must move from an infected host to a new susceptible host. Common routes include:

• Direct contact: touching infected skin or sores, or sexual contact (for example, some skin infections, HIV).
• Droplet and airborne spread: inhaling droplets from coughs and sneezes (for example, influenza, tuberculosis).
• Contaminated food and water (faecal-oral): ingesting pathogens in food or water (for example, cholera, salmonella).
• Body fluids: blood, semen and other fluids, including via shared needles or transfusion (for example, HIV, hepatitis B).
• Vectors: an organism, often an insect, that carries the pathogen between hosts (for example, mosquitoes transmitting malaria).
• Fomites: contaminated surfaces or objects (door handles, utensils) that transfer pathogens.

Factors affecting spread

The spread of an infectious disease through a population depends on the mode of transmission, the density and movement of hosts, environmental conditions (warmth and moisture favour many pathogens and vectors), the level of immunity in the population, and the standard of hygiene, sanitation and public health measures.

Why classifying pathogens matters

Knowing the type of pathogen determines how a disease is treated and controlled. Bacterial infections may respond to antibiotics; viral infections are managed with antivirals or prevented with vaccines; vector-borne diseases are controlled by targeting the vector. Identifying the mode of transmission tells public health authorities where to intervene, which is the link to epidemiology and disease control.