Pathogens and disease transmission: WACE Year 12 Human Biology Unit 3
“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”
Pathogens are disease-causing agents: bacteria, viruses, fungi, protozoa, multicellular parasites and prions, each causing characteristic diseases. They spread by modes such as contact, droplets, contaminated food or water, body fluids and vectors. Epidemiology studies how disease spreads through populations using terms such as incidence, prevalence, reservoir and the difference between an epidemic and a pandemic.
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.
A pathogen is any agent that causes disease, including bacteria, viruses, fungi, protozoa, multicellular parasites and prions. Pathogens carry antigens that the immune system can recognise as non-self, which triggers the specific immune response covered in the immune system topic.
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.
A reservoir is the natural habitat where a pathogen survives and multiplies, which may be humans, animals, soil or water. Identifying the reservoir and the mode of transmission is the key to controlling a disease, because breaking the link between the reservoir and new hosts stops the spread. For example, draining still water removes the mosquito vector breeding sites for malaria.
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.
A vector-borne disease scenario
Suppose malaria cases rise in a region after heavy rain. The pathogen is a protozoan, the reservoir is infected humans and the vector is the mosquito, which breeds in still water. Heavy rain creates more breeding sites, so the mosquito population grows and the transmission rate rises, increasing the incidence of new cases. To control the outbreak you break the transmission link: reduce the vector by draining standing water and using insecticide, protect hosts with bed nets, and treat infected people to reduce the reservoir. A marker wants you to identify the pathogen type, the mode of transmission (vector), and a control measure that targets that mode.
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.
Two errors recur. First, describing viruses as living cells; viruses are non-living particles that can only reproduce inside a host cell, which is why antibiotics do not work on them. Second, swapping incidence and prevalence: incidence is new cases over a period, prevalence is all existing cases at a point in time. Use the terms precisely when interpreting data.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2021-style6 marksPathogens can be classified into several groups. Identify four types of pathogen, name a disease caused by each, and describe two modes by which infectious diseases are transmitted between people.Show worked answer →
A 6 mark response needs four pathogen types with diseases plus two transmission modes.
Four pathogen types with diseases. Bacteria (for example tuberculosis or whooping cough), viruses (for example influenza or measles), fungi (for example tinea or thrush), and protozoa (for example malaria). Worms or prions are also acceptable with a correct disease.
Two modes of transmission. Direct contact or droplet spread, where pathogens pass through close contact or in droplets from coughing and sneezing (for example influenza). Vector transmission, where an organism such as a mosquito carries the pathogen between hosts (for example malaria). Other valid modes include contaminated food or water and contact with body fluids.
Markers reward correctly matched pathogen-disease pairs and clearly described transmission modes.
WACE 2023-style4 marksDefine the terms incidence, prevalence and reservoir, and explain how a high prevalence of a disease in a reservoir population affects the risk to others.Show worked answer →
A 4 mark answer needs three correct definitions plus the link to risk.
- Incidence
- The number of new cases of a disease in a population over a set time period.
- Prevalence
- The total number of existing cases in a population at a given time.
- Reservoir
- A host or environment where a pathogen normally lives and multiplies, from which it can spread.
- Link to risk
- If prevalence is high in a reservoir population, there are many sources of the pathogen, so the chance of contact and transmission to others rises and the disease is more likely to spread.
Markers reward the new versus total cases distinction and the reservoir as a source raising transmission.