Classify pathogens and the diseases they cause, describe modes of transmission, and use epidemiological terms such as incidence, prevalence and reservoir to describe the spread of disease

What this dot point is asking

WACE wants you to classify the agents of infectious disease, explain how each spreads, and use the language epidemiologists use to track disease. This page covers the pathogens and their transmission; the body's defences against them are covered in the immune system topic, and vaccination in the immunisation topic.

Types of pathogens

You should know the main groups and a named disease for each:

• Bacteria: living single-celled prokaryotes; cause tuberculosis, tetanus and many food-borne illnesses. Many cause damage by releasing toxins.
• Viruses: non-living particles of genetic material in a protein coat that can only reproduce inside host cells; cause influenza, measles, HIV and COVID-19.
• Fungi: include yeasts and moulds; cause tinea (athlete's foot) and thrush.
• Protozoa: single-celled eukaryotes; cause malaria and giardiasis.
• Multicellular parasites: worms and other parasites; cause tapeworm infections.
• Prions: misfolded proteins with no genetic material that cause other proteins to misfold; cause rare brain diseases.

Modes of transmission

A pathogen must get from an infected source to a new host. The main modes are:

• Direct contact: touching infected skin or lesions, including sexually transmitted infections.
• Droplet and airborne spread: coughing or sneezing releases droplets that others inhale, as with influenza and tuberculosis.
• Contaminated food and water: ingesting pathogens in unsafe food or water, as with cholera and many food-borne bacteria.
• Body fluids: blood, semen or other fluids, as with HIV and hepatitis.
• Vectors: an animal, often an insect, that carries the pathogen from host to host, such as mosquitoes spreading malaria.
• Fomites: contaminated objects or surfaces that transfer the pathogen.

Epidemiology: tracking disease in populations

Epidemiology is the study of the patterns, causes and control of disease in populations. Key terms you should use precisely:

• Incidence: the number of new cases in a population over a set period.
• Prevalence: the total number of existing cases at a given time.
• Endemic: a disease constantly present at a steady level in a population or area.
• Epidemic: a sudden rise in cases above the normal level in a region.
• Pandemic: an epidemic that spreads across countries or continents.
• Mortality and morbidity: the death rate and the rate of illness from a disease.

How this maps to the exam

Expect questions that ask you to classify a pathogen and name the disease it causes, identify the mode of transmission from a scenario, or interpret epidemiological data such as a graph of incidence over time and distinguish incidence from prevalence. Control measures usually follow logically from the mode of transmission.