How do vaccines protect a whole community, and why are antibiotics losing their power against bacteria?
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
A focused answer to the WACE Year 12 Human Biology Unit 3 dot point on disease control. How vaccines create artificial active immunity and memory cells, how herd immunity protects a population, and how antibiotics work and why resistance evolves.
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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.
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.
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.
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.