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NSWBiologySyllabus dot point

Inquiry Question 3: How can the spread of infectious diseases be controlled?

Investigate and assess the effectiveness of pharmaceuticals as treatment strategies for the control of infectious disease, including: antivirals and antibiotics, the development of antibiotic resistance, and the role of immunisation including the impact of vaccination programs in conferring herd immunity

A focused answer to the HSC Biology Module 7 dot point on pharmaceutical control of infectious disease. Covers antibiotic and antiviral mechanisms, the evolution of antibiotic resistance, vaccination types, and the herd immunity threshold with named examples.

Generated by Claude Opus 4.89 min answer

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

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
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What this dot point is asking

NESA wants you to evaluate pharmaceutical strategies for controlling infectious disease, including antibiotics, antivirals and vaccines, and to explain antibiotic resistance and herd immunity. This is one of the largest dot points in Module 7 and appears in extended responses worth 6 to 9 marks.

The answer

Pharmaceutical control of infectious disease has three main tools: antibiotics (against bacteria), antivirals (against viruses) and vaccines (preventive). Each has strengths and limitations, and each is shaped by the evolutionary biology of the pathogen.

Antibiotics

Antibiotics target structures or processes unique to bacteria, sparing host cells.

Mechanisms of action.

  • Cell wall synthesis inhibitors (penicillin, amoxicillin, cephalosporins) prevent peptidoglycan cross-linking, lysing actively growing bacteria.
  • Protein synthesis inhibitors (tetracycline, erythromycin) bind the bacterial 70S ribosome.
  • DNA replication inhibitors (fluoroquinolones such as ciprofloxacin) target bacterial DNA gyrase.
  • Folate synthesis inhibitors (sulphonamides, trimethoprim) block bacterial vitamin synthesis.

Limitations. Antibiotics do not work against viruses, fungi, protozoa or prions. Many cause side effects by killing beneficial gut bacteria.

Antivirals

Antivirals target viral-specific enzymes and life-cycle steps.

Mechanisms of action.

  • Reverse transcriptase inhibitors (zidovudine, tenofovir) block HIV's conversion of RNA to DNA.
  • Protease inhibitors (lopinavir, paxlovid) block the cleavage of viral polyproteins.
  • Neuraminidase inhibitors (oseltamivir) prevent influenza release from infected cells.
  • Polymerase inhibitors (remdesivir, sofosbuvir) block viral RNA replication.

Limitations. Antivirals are typically pathogen-specific (an HIV antiviral does not work on influenza). Resistance can develop, especially in RNA viruses with high mutation rates.

Antibiotic resistance

Origin. Random mutations in bacterial DNA occasionally produce a resistance gene (e.g. beta-lactamase that degrades penicillin). In an antibiotic-free environment, resistance confers no advantage. Once antibiotics are applied, natural selection favours resistant individuals: susceptible bacteria die, resistant bacteria reproduce.

Spread. Bacteria reproduce rapidly (every 20 minutes in good conditions) and share genes by horizontal gene transfer.

  • Conjugation. Plasmids carrying resistance genes are transferred between cells via a pilus.
  • Transformation. Bacteria take up free DNA from the environment.
  • Transduction. Bacteriophages carry resistance genes between bacterial hosts.

Consequences. Resistant infections cost lives and treatment dollars. Methicillin-resistant Staphylococcus aureus (MRSA), multi-drug resistant tuberculosis (MDR-TB) and carbapenem-resistant Enterobacteriaceae are major threats. The WHO ranks antibiotic resistance among the top ten threats to global health.

Strategies to slow resistance.

  1. Prescribe only when bacterial infection is confirmed.
  2. Complete the prescribed course.
  3. Reduce agricultural antibiotic use (banned in EU food animals as growth promoters since 2006).
  4. Invest in new antibiotic discovery (a class gap of 1987 to 2015 in approval of truly novel classes).
  5. Improve hospital infection control (handwashing, isolation of resistant cases).
  6. Surveillance programs such as Australia's AURA.

Immunisation and vaccination

Vaccines provide active artificial immunity by exposing the immune system to a pathogen antigen without causing disease, generating memory B and T cells.

Vaccine types.

  • Live attenuated (MMR, oral polio, BCG, varicella). Weakened pathogen replicates briefly. Strong immunity. Not suitable for severely immunocompromised people.
  • Inactivated (influenza, hepatitis A, rabies). Killed pathogen. Safer but often needs boosters.
  • Subunit / toxoid (tetanus, diphtheria, hepatitis B, HPV). Specific antigen or inactivated toxin.
  • mRNA (COVID-19 vaccines, in development for flu and HIV). mRNA encoding a pathogen antigen is delivered in a lipid nanoparticle. The host's cells produce the antigen and trigger immunity.
  • Viral vector (some COVID-19 and Ebola vaccines). A harmless virus delivers the pathogen antigen gene.

Herd immunity

When a high enough proportion of a population is immune, transmission chains break and even unvaccinated individuals are protected.

Threshold. The proportion of the population that must be immune is approximately 1 - 1/R0.

  • Measles (R0 = 12 to 18): 92 to 95 per cent.
  • COVID-19 ancestral strain (R0 = 2 to 3): 50 to 67 per cent. Higher for Delta and Omicron variants.
  • Polio (R0 = 5 to 7): 80 to 86 per cent.

Benefits.

  • Protects those who cannot be vaccinated (newborns, immunocompromised, severely allergic).
  • Enables eradication (smallpox 1980; polio close).
  • Reduces selection pressure for new variants.

Risks of falling below the threshold. Vaccine hesitancy or supply gaps can drop coverage below the herd immunity threshold. Measles outbreaks resurged in 2019 in parts of Europe, North America and the Pacific where coverage had fallen.

Examples in context

Example 1. Tasmanian gonorrhoea resistance crisis 2022-2023. Neisseria gonorrhoeae has progressively evolved resistance to every front-line antibiotic since penicillin in the 1940s, then ciprofloxacin in the 1990s, and most recently ceftriaxone in some strains. In 2022 and 2023, Tasmanian sexual health clinics reported clusters of ceftriaxone-resistant gonorrhoea, prompting an Australian Sexually Transmissible Infections and HIV Surveillance Report. The Royal Australian College of General Practitioners updated guidelines to require culture-based sensitivity testing before treatment in suspected resistant cases. The mechanism is mutation in the penA gene encoding the antibiotic's target penicillin-binding protein 2, plus horizontal acquisition of resistance plasmids. The case illustrates how strong directional selection (antibiotic use) and rapid bacterial evolution produce resistance within years.

Example 2. Measles vaccination and herd immunity in NSW. Measles is one of the most contagious diseases known, with a basic reproduction number (R0) of 12 to 18. To achieve herd immunity (the threshold above which an introduced case cannot sustain an outbreak), vaccination coverage must exceed 1 minus 1/R0, which means roughly 92 to 94 percent of the population must be immune. NSW Health records routine MMR (measles-mumps-rubella) coverage above 95 percent for two-year-olds. When pockets of coverage drop below 90 percent (as in some Northern NSW communities in 2014), measles outbreaks recur. The Bourke and Lismore 2014 outbreak resulted in 26 cases and demonstrated that herd immunity is fragile and locality-dependent.

Try this

Q1. Explain why antibiotics are ineffective against viral infections such as the common cold. [3 marks]

  • Cue. Viruses lack the bacterial-specific structures antibiotics target (peptidoglycan walls, 70S ribosomes); they use host cell machinery for replication.

Q2. A vaccine for a disease with R0 = 4 is rolled out in NSW. Calculate the minimum vaccination coverage required for herd immunity, and explain what happens if coverage drops to 65 percent. [3 marks]

  • Cue. Threshold is 1 - 1/4 = 75 percent. At 65 percent, R-effective exceeds 1 and outbreaks can sustain.

Q3. Evaluate strategies to combat antibiotic resistance. (a) Identify one factor driving resistance evolution. (b) Describe two specific stewardship strategies used in Australian hospitals. (c) Justify whether these strategies are sufficient to reverse current resistance trends. [1+2+3 marks]

  • Cue. (a) Overuse in human medicine and agriculture. (b) Antibiotic stewardship programs at Westmead and RPA, restricted prescribing of last-line antibiotics. (c) Stewardship slows but does not reverse; new antibiotics and global coordination are essential.

Exam-style practice questions

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

2023 HSC5 marksTetanus vaccines were introduced in 1953, reducing case numbers, with most cases now in people aged 65 and over. A graph shows the tetanus vaccination schedule (antibody level rising above the 'immune' threshold after each of 5 doses given through childhood). Assess the use of vaccinations and the vaccination schedule. Use the data provided to support your answer.
Show worked answer →

Top band (5) needs a comprehensive, data-supported judgement. Key points from the guidelines:

  • Vaccinations initiate an immune response using a form of the pathogen/antigen that does not cause infection.
  • From the graph, the more booster shots, the longer the person stays immune (each dose lifts antibody level above the immune threshold).
  • Cases occurring in people over 65 result from not having a booster for a long time (immunity wanes).
  • The reduction in tetanus cases shows vaccinations are effective, while the recurring schedule maintains immunity.

Judgement: vaccinations and the booster schedule are effective/valuable. Marker feedback: make an actual judgement (don't just describe the graph) and use all the stimulus.

2022 HSC5 marksJelly Bush honey has a high level of methylglyoxal, known to help fight infection. Design a safe procedure that a scientist could use in a laboratory to investigate the effectiveness of Jelly Bush honey as a pharmaceutical to inhibit bacterial growth, using agar plates.
Show worked answer →

Full marks (5) require a valid, safe, controlled agar-plate procedure. A strong design includes:

  • Inoculate agar plates with a pure culture of a known bacterium (spread evenly).
  • Apply discs/wells soaked in honey at one or more concentrations; include a control (e.g. a disc with no honey, or sterile water) for comparison.
  • Control variables: same bacterial species/concentration, agar type, plate size, incubation temperature and time.
  • Incubate (e.g. 24–48 h) and measure the zone of inhibition (clear area) around each disc; repeat for reliability.
  • Safety: work aseptically, sterilise equipment, seal plates and do not reopen them after incubation, wear gloves, disinfect the bench.

Marker feedback: inoculate with a pure culture, compare to a control, and avoid opening dishes after incubation.

2021 HSC7 marksA study compared 8134 children who received the measles vaccine with 8134 unvaccinated children (matched for age, sex, dwelling, siblings, maternal education). Graphs show measles cases in each group, and a table shows deaths by cause (measles, diarrhoea/dysentery, oedema, fever) for both groups. 'A vaccine only protects the community against a specific disease.' Analyse the data with reference to this statement.
Show worked answer →

Top band (7) needs analysis of the data with arguments for and against the statement. Key points from the guidelines:

  • Supports the statement (specific protection): the measles vaccine greatly reduces measles — incidence in vaccinated children is very low/zero vs unvaccinated, and only 2 vaccinated children died of measles vs 40 unvaccinated. Since groups were matched, the vaccine is likely responsible.
  • Against the statement (non-specific benefits): vaccinated children also died less from other diseases — about half the rate from diarrhoea/dysentery and under one-third from oedema — suggesting protection beyond the specific disease. But oedema numbers are small, and fever differences are minor, so protection is not uniform.
  • Overall: the vaccinated group had about half the total mortality, so the statement requires qualification.

Marker feedback: use all the stimulus and manipulate the data to build arguments both for and against.

2020 HSC3 marksOutline a benefit and a limitation of using pharmaceuticals such as antibiotics to treat infectious disease.
Show worked answer →

3 marks for outlining both a benefit and a limitation. Sample answer: Benefit — antibiotics treat bacterial infections by inhibiting bacterial growth (or killing bacteria). Limitation — antibiotic resistance in bacteria is becoming increasingly common, reducing the effectiveness of many antibiotics. 2 marks for identifying both, or outlining only one. Marker feedback: match the drug to the right pathogen (antibiotics work on bacteria, not viruses); don't confuse antibiotics with antibodies; and note that the bacteria (not the patient's body) develop resistance.

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