Immunisation and disease control: WACE Year 12 Human Biology Unit 3
“Explain how immunisation produces artificial active immunity, how herd immunity protects a population, and how antibiotic use and resistance affect the control of infectious disease”
Immunisation introduces antigens through a vaccine so the body makes its own antibodies and memory cells (artificial active immunity) without serious illness, giving a fast, strong secondary response on real exposure. When enough people are vaccinated, herd immunity protects the unvaccinated by reducing transmission. Antibiotics kill bacteria, but overuse selects for resistant strains, so resistance spreads.
What this dot point is asking
WACE wants you to connect the immune response to public health: how vaccines work at the individual and population level, and how antibiotics and resistance shape disease control. This builds directly on the lines of defence and active versus passive immunity from the immune system topic.
How vaccines work
A vaccine contains antigens from a pathogen in a form that does not cause serious disease, such as a weakened (attenuated) or dead pathogen, a fragment of it, or its genetic instructions. When injected, these antigens trigger the specific immune response: B cells are activated, plasma cells make antibodies, and crucially memory B and T cells are produced. The person now has artificial active immunity, having made their own antibodies and memory cells, but without having had the disease.
If the person later meets the real pathogen, the memory cells trigger a rapid, large secondary response that destroys the pathogen before it can cause illness. A booster dose re-exposes the immune system to the antigen, increasing the number of memory cells and extending protection.
Vaccination produces active immunity because the person's own immune system responds to the antigens and makes memory cells, so protection is long-lasting. This is different from passive immunity (ready-made antibodies from another source, such as across the placenta), which is immediate but temporary because no memory cells form. A vaccine works slowly to build protection but lasts.
Herd immunity
When a high proportion of a population is immune (through vaccination or past infection), the pathogen cannot easily find new susceptible hosts, so chains of transmission break. This is herd immunity, and it protects people who cannot be vaccinated, such as newborns or the immunocompromised, because they are unlikely to meet an infected person. The proportion needed depends on how contagious the disease is; highly contagious diseases such as measles need very high vaccination rates.
Herd immunity is the protection a population gains when a large enough proportion of its members are immune that the pathogen can no longer spread efficiently. Even unvaccinated individuals are protected because the chain of transmission is broken. It is a population-level effect, not an individual one.
Antibiotics and how they work
Antibiotics are chemicals that kill bacteria or stop them reproducing, for example by disrupting the bacterial cell wall. They are effective only against bacteria, not viruses, because viruses lack the structures antibiotics target and reproduce inside host cells. This is why antibiotics do not treat colds or influenza, which are viral.
Antibiotic resistance
Antibiotic resistance is a clear example of natural selection. Within a bacterial population there is variation, and by chance a few bacteria carry an allele that makes them resistant to an antibiotic. When the antibiotic is used, non-resistant bacteria die but resistant ones survive (differential survival). The survivors reproduce and pass on the resistance allele, so the proportion of resistant bacteria increases over generations. Overuse and misuse of antibiotics, including not finishing a course, speed this up by giving resistant bacteria more chances to be selected.
Why finishing the full course matters
A bacterial population varies in resistance to an antibiotic, with a few resistant cells present by chance from mutation. When an antibiotic is taken, susceptible bacteria are killed quickly while resistant ones survive longer. If the course is stopped early, the most resistant survivors are not killed; they reproduce and pass on the resistance allele, so the next generation has a higher frequency of resistance. Repeated across many people, this selects for resistant strains that the antibiotic can no longer treat. A marker wants you to frame this as natural selection: variation exists, the antibiotic is the selection pressure, resistant bacteria survive and reproduce, and resistance allele frequency rises.
How this maps to the exam
Expect questions that ask you to explain how a vaccine produces immunity (link to memory cells and the secondary response), why vaccines give active not passive immunity, how herd immunity protects a community, or to explain antibiotic resistance as natural selection. The resistance question is a strong link forward to the population genetics and selection content in Unit 4.
Two errors recur. First, claiming antibiotics work on viruses; they act only on bacteria, so they cannot treat colds, influenza or other viral illnesses. Second, describing bacteria as deliberately developing resistance; resistance arises from random mutation and is then selected for by antibiotic use. The bacteria do not change in response to need; the antibiotic selects for resistant variants that already exist.
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 marksExplain how a vaccine produces artificial active immunity, and explain why a person who is vaccinated responds more quickly to a later infection by the same pathogen.Show worked answer →
A 6 mark explain response needs the primary response to the vaccine and the secondary response on re-exposure.
- What the vaccine contains
- A vaccine contains a weakened, dead or fragment form of the pathogen carrying its antigens, but it cannot cause the disease.
- Primary response
- The antigens are recognised by B lymphocytes, which are selected and divide. Plasma cells produce antibodies and memory B cells are formed. Because the body makes its own antibodies, this is active immunity, and because it is triggered by a vaccine rather than natural infection it is artificial.
- Secondary response
- The memory cells remain in the body. On a later infection by the same pathogen, memory cells recognise the antigen and divide rapidly into plasma cells, so antibodies are produced faster and in greater quantity. The pathogen is destroyed before symptoms develop.
Markers reward memory cell formation and the faster, larger secondary response.
WACE 2024-style4 marksExplain how herd immunity protects unvaccinated individuals, and explain how the overuse of antibiotics leads to antibiotic-resistant bacteria.Show worked answer →
A 4 mark answer needs the herd immunity mechanism and the selection mechanism for resistance.
Herd immunity. When a high proportion of a population is immune, there are few susceptible hosts, so a pathogen cannot spread easily. This breaks transmission chains and protects unvaccinated individuals because they are unlikely to come into contact with an infected person.
Antibiotic resistance. In a bacterial population, variation means a few bacteria carry resistance. When antibiotics are overused, the susceptible bacteria are killed but the resistant ones survive and reproduce, passing on the resistance. Over time the resistant form dominates, a clear case of natural selection.
Markers reward fewer susceptible hosts breaking transmission and the survival and reproduction of resistant variants.