Explain epidemiological terms and evaluate strategies used to control the spread of disease

A focused answer to the WACE Year 12 Biology dot point on epidemiology and disease control. Covers epidemics and pandemics, transmission rate, herd immunity, quarantine, vaccination programs and antibiotic resistance.

Generated by Claude Opus 4.88 min answerUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking

SCSA wants you to use epidemiological vocabulary correctly and to evaluate control strategies, weighing how each interrupts transmission. A strong answer links a control measure to the specific step in the transmission cycle it targets.

What epidemiology studies

Epidemiology is the study of the patterns, causes and control of disease in populations. Epidemiologists track who gets a disease, where, when and why, using data on incidence and prevalence to design and assess interventions.

Key terms for describing disease in a population:

Endemic: a disease constantly present at a relatively stable level in a particular area (for example, malaria in parts of the tropics).
Epidemic: a sudden increase in the number of cases above the normal level in a region.
Pandemic: an epidemic that spreads across countries or continents (for example, the COVID-19 pandemic).
Incidence: the number of new cases in a period.
Prevalence: the total number of existing cases at a given time.
Mortality rate: the proportion of a population that dies from the disease.
Morbidity: the level of illness or disability caused by a disease.

Transmission and the chain of infection

Disease spreads along a chain: a pathogen leaves an infected host, travels by some mode of transmission, and enters a new susceptible host. Control strategies work by breaking a link in this chain. The rate of spread depends on how easily the pathogen is transmitted, how many susceptible people there are, and how mobile and densely packed the population is.

A useful measure is the basic reproduction number (R0), the average number of new infections caused by one infected person in a fully susceptible population. If the effective reproduction number is kept below one, each case leads to fewer than one new case on average and the outbreak shrinks.

Control strategies

You should be able to explain and evaluate a range of strategies and identify which link they target.

Hygiene and sanitation: hand washing, clean water and safe food handling reduce faecal-oral and contact transmission.
Quarantine and isolation: separating infected or exposed individuals stops them passing the pathogen to others. Quarantine applies to those exposed but not yet symptomatic; isolation applies to confirmed cases.
Physical measures: masks, physical distancing and improved ventilation reduce droplet and airborne spread.
Vector control: removing mosquito breeding sites, insecticides and bed nets reduce vector-borne transmission such as malaria.
Vaccination programs: immunising the population reduces the number of susceptible hosts.
Surveillance and contact tracing: identifying cases and their contacts allows targeted isolation before further spread.
Public health education: informing people about transmission and prevention changes behaviour.

Herd immunity

Herd immunity occurs when a large enough proportion of a population is immune (through vaccination or prior infection) that the pathogen cannot spread easily, because most contacts an infected person has are with immune individuals. This indirectly protects those who are not immune, such as newborns or people who cannot be vaccinated.

The proportion needed for herd immunity is higher for more contagious diseases. Falling vaccination rates can drop a population below this threshold, allowing outbreaks of diseases that were previously controlled.

Antibiotic resistance as a control challenge

The overuse and misuse of antibiotics has driven the evolution of resistant bacteria by natural selection: antibiotics kill susceptible bacteria, leaving resistant ones to reproduce and spread their resistance alleles. Strategies to slow resistance include prescribing antibiotics only when needed, completing prescribed courses, and developing new drugs. This links Unit 4 disease control back to the Unit 3 idea of selection.

Why monitoring and control matter

Tracking incidence and prevalence lets authorities detect outbreaks early, target resources, and judge whether interventions are working. Because no single measure is perfect, control usually combines several strategies that each break a different link in the chain, reducing the effective reproduction number until the outbreak fades.

Exam-style practice questions

Practice questions written in the style of SCSA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

WACE 20226 marksExplain what is meant by herd immunity, and explain why a higher proportion of a population must be immune to achieve herd immunity for a more contagious disease.

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A 6 mark answer needs the mechanism plus the contagiousness link.

Herd immunity. When a large enough proportion of a population is immune (through vaccination or prior infection), the pathogen cannot spread easily, because most contacts of an infected person are with immune individuals who do not pass it on. This indirectly protects those who are not immune (such as newborns or people who cannot be vaccinated).

Why more contagious diseases need a higher threshold. A more contagious disease has a higher basic reproduction number ( $R_0$ ): each infected person would, on average, infect more people in a fully susceptible population. To stop spread, the effective reproduction number must be pushed below one, so a larger fraction of contacts must be immune. The more people each case would otherwise infect, the higher the proportion that must be immune to interrupt transmission.

Markers reward the indirect-protection mechanism and the link between higher $R_0$ and a higher immune threshold.

WACE 20245 marksQuarantine and vaccination are both used to control infectious disease. Compare how each strategy interrupts the chain of transmission, and evaluate one limitation of each.

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A 5 mark answer needs the mechanism of each plus a limitation of each.

Quarantine: Separates exposed individuals during the incubation period, so any who become infectious cannot pass the pathogen to the wider community. It targets the transmission link directly. Limitation: it is costly and hard to enforce, and fails if people spread the disease before symptoms appear.
Vaccination: Makes individuals immune, reducing the number of susceptible hosts so the pathogen has fewer people to infect. It targets the susceptible-host link. Limitation: it requires an available, effective vaccine and high uptake; some people cannot be vaccinated, and low coverage prevents herd immunity.
Comparison: Quarantine acts on already-exposed individuals to block onward spread; vaccination acts in advance to remove susceptibles.

Markers reward the distinct transmission link each targets and a valid limitation of each.