Infectious Diseases
Four named diseases that have shaped human history and continue to cause significant global mortality. The biology of pathogens, the routes of transmission, and the biological, social, and economic factors that make disease control a problem of both biology and public health policy. Plus how penicillin works, and the growing challenge of antibiotic resistance.
Infectious diseases
An infectious disease is a disease caused by a pathogen — a microorganism that invades the body and causes harm. By definition, infectious diseases are transmissible: they can pass from one host to another, directly or indirectly. This distinguishes them from non-infectious diseases such as cardiovascular disease, type 2 diabetes, or most cancers, which are not caused by pathogens and cannot be passed between individuals.
For 9700 (2025–2027), the four named diseases are cholera, malaria, tuberculosis (TB), and HIV/AIDS. The syllabus explicitly states that details of the life cycle of the malarial parasite are NOT expected. Earlier 9700 syllabuses included measles and smallpox; these have been removed for 2025–2027 and are not assessable.
Types of pathogen
| Type | Cell type | Reproduction | Antibiotic-treatable? | Example diseases |
|---|---|---|---|---|
| Bacteria | Prokaryotic | Binary fission | Yes | Cholera, TB |
| Viruses | Non-cellular | Inside host cells only | No | HIV/AIDS, influenza, COVID-19 |
| Protoctists | Eukaryotic, single-celled | Various; often inside host cells | No (need antiprotozoal drugs) | Malaria |
| Fungi | Eukaryotic | Spores; budding | No (need antifungal drugs) | Athlete's foot, ringworm (not in 9700) |
Each disease has a particular pathogen and a particular transmission route. Understanding both is the basis for prevention and control.
Cholera
Pathogen: the bacterium Vibrio cholerae.
Toxigenic strains produce cholera toxin, which causes intestinal cells to release large quantities of chloride ions into the gut lumen. Water follows by osmosis, producing severe watery diarrhoea (sometimes called “rice-water stools”). Without treatment, dehydration can be fatal within hours of severe symptom onset.
Faecal–oral route: the bacterium is shed in the faeces of infected people and consumed via contaminated water or food. This is fundamentally a waterborne disease:
- Drinking water contaminated with sewage
- Food (especially seafood) prepared with contaminated water
- Contact transmission via unwashed hands
Outbreaks occur where clean water and sanitation infrastructure are damaged or absent — including disaster zones, conflict-affected regions, and refugee settings.
Malaria
Four protoctist species in the genus Plasmodium cause human malaria:
- P. falciparum — the most severe form; responsible for most malaria deaths
- P. vivax
- P. ovale
- P. malariae
The parasite invades red blood cells, multiplies inside them, and lyses them — causing the cyclical fevers and anaemia characteristic of malaria.
Vector-borne: spread between humans by the female Anopheles mosquito (only the female takes blood meals; she needs blood for egg development).
- An infected mosquito injects parasites into the bloodstream when it bites
- The mosquito picks up parasites from infected humans during biting; the parasite develops inside the mosquito before becoming infectious to the next person
- Direct human-to-human transmission can occur via blood transfusion or shared needles, but is rare; mother-to-child transmission across the placenta is also possible but uncommon
Endemic in many tropical and subtropical regions where the Anopheles mosquito flourishes (warm climate; standing water for breeding).
The 9700 syllabus explicitly does NOT require detailed knowledge of the Plasmodium life cycle inside the mosquito or the human host (sporozoites, merozoites, gametocytes, liver vs blood stages). For 10.1 you only need: pathogen identity, transmission via female Anopheles mosquito, and biological/social/economic factors for control.
Tuberculosis (TB)
Two bacterial species:
- Mycobacterium tuberculosis — causes most cases; primarily a human disease
- Mycobacterium bovis — primarily a cattle disease that can also infect humans, historically through unpasteurised milk from infected cattle (largely controlled in regions with milk pasteurisation)
Both bacteria infect the lungs, where they form lesions called tubercles. The infection can become latent (no symptoms, not contagious) and reactivate later, especially when the immune system is weakened.
Airborne droplets from people with active pulmonary TB:
- Coughing, sneezing, talking — all release droplet nuclei containing bacteria
- Close, prolonged contact in enclosed, poorly ventilated spaces is the main risk — e.g. household contacts of an infected person
- Brief casual contact carries low risk
- Historical: M. bovis from drinking unpasteurised milk — now rare where pasteurisation and cattle screening are routine
Risk of progression from infection to active TB is greatly increased by HIV co-infection, malnutrition, smoking, and diabetes.
HIV/AIDS
Pathogen: the human immunodeficiency virus (HIV) — a retrovirus that infects T-helper lymphocytes (Topic 11). The virus integrates into host cell DNA and gradually destroys the T-helper cell population.
Over years (typically 5–15 without treatment), the immune system becomes severely impaired. AIDS (Acquired Immunodeficiency Syndrome) is the late-stage condition defined by very low T-helper cell counts and the appearance of opportunistic infections (e.g. Pneumocystis pneumonia, TB) and certain cancers that a healthy immune system would suppress.
Modern antiretroviral therapy (ART) suppresses viral replication and allows people with HIV to live near-normal lifespans — HIV is now a manageable chronic condition rather than the rapid death sentence it was in the 1980s.
HIV is transmitted through specific routes only, by transfer of body fluids (blood, semen, vaginal fluids, breast milk):
- Unprotected sexual intercourse with an infected person (any type of unprotected sexual contact involving fluid exchange)
- Blood-to-blood contact: sharing needles or other injecting equipment; transfusion of unscreened infected blood; needlestick injuries (occupational risk for healthcare workers)
- Mother to child: across the placenta during pregnancy, during childbirth, or via breast milk during breastfeeding
HIV is NOT transmitted by: shaking hands, hugging, sharing food or cutlery, mosquito bites, sneezing or coughing, toilet seats, or swimming pools. These misconceptions have caused enormous unnecessary stigma toward people living with HIV.
HIV affects people across all communities, regions, and demographics. There is no “type” of person who gets HIV. Exam questions should be answered on the basis of the biology of transmission and the social/economic conditions that affect access to prevention and treatment — not on assumptions about any group of people. Use language like “people living with HIV”.
Four-disease comparison — the AS-level minimum
| Disease | Pathogen | Pathogen type | Transmission | Main affected system |
|---|---|---|---|---|
| Cholera | Vibrio cholerae | Bacterium | Faecal-oral via contaminated water/food | Small intestine |
| Malaria | Plasmodium falciparum, P. malariae, P. ovale, P. vivax | Protoctists | Vector-borne via female Anopheles mosquito | Red blood cells; liver |
| Tuberculosis | Mycobacterium tuberculosis, M. bovis | Bacteria | Airborne droplets from infected person; historically M. bovis via unpasteurised milk | Lungs (and other organs) |
| HIV/AIDS | Human immunodeficiency virus | Virus (retrovirus) | Body fluid exchange: sexual contact, blood-to-blood, mother to child | Immune system (T-helper cells) |
Biological, social, and economic factors in prevention and control
The 9700 syllabus requires you to discuss prevention and control through three lenses: biological, social, and economic. The same disease will have different control challenges in different settings — structural answers should address all three categories.
Biological: oral rehydration therapy (ORS) is simple and highly effective; antibiotics for severe cases; oral cholera vaccines (provide useful temporary protection in outbreaks).
Social: public health education on hand-washing, water safety, latrine use; emergency response infrastructure for outbreaks; vaccination of refugees and travellers in high-risk regions.
Economic: investment in clean water supply and sewage treatment is the single most effective long-term measure but requires substantial infrastructure spending; outbreak response costs are high in regions with limited public-health budgets.
Biological: insecticide-treated bed nets; indoor residual spraying; antimalarial drugs for treatment (artemisinin-based combination therapy — ACT) and prophylaxis for travellers; emerging vaccines (RTS,S/AS01 deployed in some regions).
Social: education on bed-net use and preventive measures; community-led elimination of mosquito breeding sites (drainage of standing water); cross-border cooperation on outbreak control.
Economic: bed-net distribution programmes; cost of antimalarial drugs (resistance to chloroquine, and increasing resistance to artemisinin in some regions, drives up costs); investment in vector-control infrastructure; impact of malaria on workforce productivity in endemic regions.
Biological: BCG vaccine (limited and variable effectiveness, especially in adults); standard 6-month antibiotic course (isoniazid, rifampicin, pyrazinamide, ethambutol); contact tracing and screening; treatment of latent infection in high-risk individuals; pasteurisation of milk and cattle screening for M. bovis.
Social: overcrowded housing increases transmission — addressing through housing policy; HIV testing and treatment (since HIV co-infection greatly increases TB risk); reducing stigma so people seek treatment promptly; directly observed therapy (DOT) programmes to ensure completion of antibiotic courses.
Economic: long treatment courses are expensive and require sustained healthcare access; multi-drug resistant TB (MDR-TB) requires much longer, costlier treatment; impact on workforce productivity.
Biological: antiretroviral therapy (ART) suppresses viral load to undetectable levels; pre-exposure prophylaxis (PrEP) for people at risk; condoms for sexual transmission; sterile needle exchange programmes; mother-to-child transmission prevention through ART during pregnancy and infant feeding strategies; donated blood screening; no fully effective HIV vaccine has yet been developed (as of writing).
Social: public health education on transmission and prevention; reducing stigma to encourage testing and treatment; targeted programmes for high-risk groups; integrating HIV services with reproductive health services.
Economic: ART is now widely subsidised but reaching all who need it remains a global challenge; infrastructure for confidential testing and lifelong treatment requires substantial healthcare investment.
For an extended question on any of the four diseases, organise around the syllabus framework:
- Briefly identify the pathogen and main transmission route
- Cover biological factors: medical interventions (vaccines, drugs, vector control)
- Cover social factors: education, behaviour change, infrastructure, healthcare access
- Cover economic factors: resource constraints, cost-effectiveness, productivity loss
- Conclude with the most effective intervention and any remaining barriers
This three-factor structure tracks the syllabus LO directly and prevents you from giving an answer that is purely biological or purely social.
Which of the following correctly pairs a named disease with its pathogen type?
- A. Malaria — bacterium
- B. Cholera — virus
- C. Tuberculosis — bacterium
- D. HIV/AIDS — protoctist
A region with limited sanitation infrastructure is experiencing a cholera outbreak following severe flooding.
(a) State the pathogen that causes cholera and explain how the disease is normally transmitted. [3]
(b) Discuss the biological, social, and economic factors that affect the prevention and control of cholera in this region. [6]
(a) Cholera pathogen and transmission [3 marks]
- Cholera is caused by the bacterium Vibrio cholerae [1]
- Transmitted by the faecal-oral route — the bacterium is shed in the faeces of infected people [1]
- Spread mainly via contaminated drinking water (or food prepared with contaminated water) [1]
(b) Three-factor discussion [6 marks; 2 marks per factor]
- Treat severe cases with oral rehydration solution (ORS) and IV fluids; antibiotics in severe cases [1]
- Oral cholera vaccines can be deployed during outbreaks; provide temporary protection while permanent infrastructure is built [1]
- Public health education on hand-washing, food and water safety, safe disposal of waste; reduce contact with contaminated water [1]
- Coordinate with humanitarian organisations to deliver temporary safe-water supplies and sanitation facilities [1]
- The most effective long-term solution — clean water supply and sewage treatment infrastructure — requires major investment beyond the means of the local economy in disaster conditions [1]
- Outbreak response (vaccines, ORS, healthcare staff) costs may need international aid; the disruption also reduces local economic productivity, deepening the cycle [1]
Mark scheme guidance: “Discuss” questions reward balanced coverage across all three categories. Answers focused only on biological factors typically lose half the available marks.
Antibiotics
An antibiotic is a chemical substance that kills bacteria or inhibits their growth. The discovery of penicillin (Fleming, 1928) and its therapeutic development in the 1940s transformed medicine: previously untreatable bacterial infections such as bacterial pneumonia, syphilis, and post-surgical infection became routinely curable. The growing problem of antibiotic resistance now threatens to undo much of that progress.
How penicillin acts on bacteria
Penicillin and its derivatives target a feature unique to bacteria: their cell wall.
- Bacterial cell walls are made of peptidoglycan (also called murein) — a mesh-like polymer of sugar chains cross-linked by short peptide bridges
- The cross-linking is performed by enzymes (transpeptidases) during cell wall synthesis as bacteria grow and divide
- Penicillin inhibits these cross-linking enzymes, so newly made cell wall lacks the cross-links needed for strength
- The weakened cell wall cannot withstand the high internal osmotic pressure of the bacterial cell
- Water enters the bacterium by osmosis (its cytoplasm is hypertonic to the surroundings); the cell ruptures (lyses) and dies
- Penicillin only affects actively growing bacteria — cells with already-complete cell walls are not killed unless they are dividing and need to make new wall material
Human cells have no cell wall — only a phospholipid plasma membrane. They have no peptidoglycan and no cross-linking enzymes for penicillin to inhibit. So penicillin is highly selectively toxic: it harms bacteria but spares human cells. This selective toxicity is the basis of safe antibiotic therapy.
Why antibiotics do not affect viruses
Antibiotics target features specific to bacterial cells — cell walls, prokaryotic ribosomes, bacterial DNA replication enzymes, bacterial folate synthesis. Viruses share none of these:
Viruses are not cells. They are typically just a protein coat (capsid) around a nucleic acid (DNA or RNA), sometimes with a lipid envelope. No peptidoglycan means no target for penicillin or related cell-wall antibiotics.
Viruses have no ribosomes, no metabolic enzymes, no respiration. Antibiotics that target bacterial protein synthesis (e.g. tetracycline, streptomycin) or bacterial folate synthesis have nothing to attack in a virus.
Viruses replicate by hijacking host cell ribosomes and enzymes — making selective targeting extremely difficult, because anything that inhibits virus replication in the host cell may also damage the host cell. Antiviral drugs (a separate class from antibiotics) target very specific virus-only steps such as reverse transcription in HIV.
How antibiotic resistance arises — mutation and selection
Antibiotic resistance is a textbook example of natural selection happening on a human timescale.
- In any bacterial population, random mutations occasionally produce variants with reduced sensitivity to a particular antibiotic — for example, an enzyme mutation that breaks down penicillin (penicillinase / β-lactamase), or a slight change in the antibiotic's target
- These resistance mutations exist before the antibiotic is used — they are random and pre-existing, not produced by the drug
- When the antibiotic is applied, it kills sensitive bacteria but resistant variants survive and continue to reproduce — the antibiotic acts as a selection pressure
- The resistant bacteria pass the resistance allele to their offspring (vertical transmission); they may also share it with unrelated bacteria via plasmids (horizontal gene transfer)
- Over generations, the resistant variant becomes the dominant form of the bacterium; the antibiotic loses effectiveness
Consequences of antibiotic resistance
- Treatment failure: previously routine bacterial infections become difficult or impossible to treat with first-line antibiotics
- Increased mortality and morbidity: drug-resistant infections lead to longer illness, more complications, and higher death rates
- Multi-drug resistant (MDR) and extensively drug resistant (XDR) strains: organisms such as MDR-TB are resistant to multiple first-line drugs, requiring much longer (1.5–2 years), costlier, more toxic regimens; XDR-TB is resistant to even more drugs
- Higher healthcare costs: prolonged hospital stays, expensive second-line drugs, intensive care needs
- Spread of resistance: resistant bacteria can spread within hospitals (e.g. MRSA — methicillin-resistant Staphylococcus aureus) and into the community
- Pre-antibiotic era risks return: if resistance becomes widespread, surgery, chemotherapy, organ transplants, and intensive care all become much riskier — these procedures depend on prophylactic and therapeutic antibiotics to prevent and treat infections
Steps to reduce antibiotic resistance
Doctors should not prescribe antibiotics for viral infections (e.g. common cold, most sore throats); patients should not pressure for them. Diagnostic tests should be used where available to identify true bacterial infections.
Patients should finish the full course of antibiotics even if they feel better — stopping early leaves a population of partially-resistant bacteria that can multiply. Take at the prescribed intervals to maintain effective concentrations.
Antibiotic use in livestock for growth promotion or routine prophylaxis is a major source of resistance. Restrict to genuine veterinary therapeutic need; many regions are phasing out routine agricultural use.
Hand-washing, isolation of infected patients, sterilisation of equipment, and screening on hospital admission all reduce hospital-acquired infections, lowering the demand for antibiotics and reducing transmission of resistant strains.
Pharmaceutical research into new antibiotic classes — though commercial incentives are weak (antibiotics are taken short-term and resistance rapidly limits their useful life). Public funding and antibiotic stewardship policies are increasingly important.
Public understanding of when antibiotics work (and when they don't), of the importance of finishing courses, and of the global stakes of resistance. National antibiotic stewardship programmes monitor and guide prescribing.
Antibiotics such as penicillin are not effective against viral infections such as influenza or COVID-19. Why?
- A. Viruses reproduce too rapidly for antibiotics to keep up
- B. Viruses lack the cellular structures (e.g. cell walls and ribosomes) that antibiotics target
- C. Viral infections are usually mild and don't need antibiotics
- D. The body's own immune system always destroys viruses before antibiotics can act
Penicillin is a widely used antibiotic, but its effectiveness has been reduced by the spread of resistant bacterial strains.
(a) Outline how penicillin acts on bacteria. [3]
(b) Explain how a population of bacteria can become resistant to penicillin, with reference to mutation and selection. [4]
(c) Suggest two steps that can be taken to slow the spread of antibiotic resistance, and explain how each helps. [2]
(a) Penicillin mechanism [3 marks]
- Penicillin inhibits the enzymes that cross-link peptidoglycan during bacterial cell wall synthesis [1]
- Newly made cell wall is weakened and cannot resist the bacterium's high internal osmotic pressure [1]
- Water enters the cell by osmosis and the bacterial cell ruptures (lyses) and dies; only actively growing bacteria are affected [1]
(b) Mutation + selection [4 marks]
- Random mutations in the bacterial genome occasionally produce variants with reduced sensitivity to penicillin (e.g. producing the enzyme penicillinase / β-lactamase that destroys penicillin) [1]
- These mutations exist before the antibiotic is used — they are not caused by the antibiotic [1]
- When penicillin is applied, sensitive bacteria are killed but resistant variants survive — the antibiotic provides a selection pressure [1]
- The resistant variants reproduce rapidly and pass resistance to offspring (and may share resistance plasmids with unrelated bacteria) — over generations the resistant strain becomes dominant [1]
(c) Two steps [2 marks; 1 each, must explain mechanism]
- Prescribe antibiotics only for genuine bacterial infections — reduces selection pressure on bacteria so resistance does not spread as fast
- Patients complete the full prescribed course — ensures all bacteria are killed; partial courses leave less-sensitive variants alive to reproduce
- Restrict antibiotic use in agriculture — reduces the volume of antibiotics in the environment, lowering the selection pressure on bacterial populations
- Improve infection control in hospitals — prevents resistant strains from spreading between patients
- Develop new antibiotics — provides alternatives once existing drugs lose effectiveness
Topic 10 Practice — Comprehensive
Mixed practice covering both sub-sections in 9700 P1/P2 style. Try each before revealing the answer.
Which combination of pathogen and disease is correct?
- A. Plasmodium falciparum — tuberculosis
- B. Mycobacterium tuberculosis — cholera
- C. Vibrio cholerae — cholera
- D. Human immunodeficiency virus — malaria
Which of the following is NOT a route of HIV transmission?
- A. Mother to child during pregnancy or breastfeeding
- B. Sharing needles between people who inject drugs
- C. Unprotected sexual intercourse with an infected person
- D. Sharing food, hugging, or shaking hands
Penicillin kills bacteria by interfering with which structure or process?
- A. The bacterial plasma membrane
- B. The cross-linking of peptidoglycan in the bacterial cell wall
- C. The bacterial 80S ribosome
- D. The DNA replication machinery of the host cell
A previously effective antibiotic has become much less useful for treating a particular bacterial infection over the past 15 years.
(a) Explain, in terms of mutation and selection, how the bacterial population has become resistant. [4]
(b) Suggest TWO biological reasons why some bacteria are particularly likely to evolve resistance rapidly. [2]
(c) Explain why doctors should not prescribe antibiotics for a viral infection like the common cold, even if the patient asks for them. [2]
(a) Mutation + selection [4 marks]
- Random mutations in bacterial DNA occasionally produce variants with reduced sensitivity / a resistance gene (e.g. producing an enzyme that breaks down the antibiotic) [1]
- These mutations occur before antibiotic exposure — they are pre-existing variation, not produced by the drug [1]
- When the antibiotic is used, sensitive bacteria are killed but resistant variants survive (selection pressure favours them) [1]
- Resistant bacteria reproduce, passing resistance to offspring (vertical transmission) and sometimes to unrelated bacteria via plasmids; over many generations the resistant form dominates the population [1]
(b) Why bacteria evolve resistance rapidly [2 marks]
- Bacteria reproduce extremely rapidly (binary fission, ~20 minutes per generation in ideal conditions) — many generations per day, so mutations accumulate quickly [1]
- Bacterial populations are very large, so even rare mutations are present in some individuals [1]
- Bacteria can transfer resistance genes between cells (and between species) via plasmids — horizontal gene transfer spreads resistance faster than mutation alone [1]
- High mutation rate per generation in some bacteria [1]
(c) Antibiotics not for viral infections [2 marks]
- Antibiotics target features specific to bacteria (e.g. cell wall, prokaryotic ribosomes); viruses lack these structures, so antibiotics have no effect on them [1]
- Unnecessary antibiotic use exposes the body's bacterial flora to the drug, providing selection pressure that drives the evolution of resistance — reducing the antibiotic's effectiveness when it is genuinely needed [1]
Synoptic note: This question links Topic 10 to natural selection (Topic 17 at A2) and to bacterial cell biology (Topic 1).
Discuss the biological, social, and economic factors involved in the prevention and control of malaria. [8]
Strong answers organise around the three syllabus categories. Award marks across categories for breadth.
- Insecticide-treated bed nets — physically prevent female Anopheles mosquitoes biting people while they sleep (the main biting time)
- Indoor residual spraying with insecticide reduces mosquito populations
- Antimalarial drugs — artemisinin-based combination therapy (ACT) for treatment; chemoprophylaxis for travellers and high-risk groups
- Emerging vaccines (e.g. RTS,S/AS01) deployed in some regions
- Elimination of breeding sites — drainage of standing water
- Education on bed-net use, recognising symptoms, and seeking early treatment
- Community-based programmes for breeding-site elimination
- Training community health workers to diagnose and treat malaria locally where hospital access is limited
- Cross-border cooperation — mosquitoes and people both move across national boundaries
- Bed-net distribution and indoor spraying programmes require sustained funding
- Resistance to chloroquine, and increasing resistance to artemisinin in some regions, drives up the cost of treatment as new drugs are required
- Vector-control infrastructure (drainage, environmental management) requires large public investment
- Malaria reduces workforce productivity and increases healthcare burden in endemic regions, depressing economic growth and creating a cycle that is difficult to escape without external assistance
Up to 8 marks for substantive points spread across all three categories. Discussions covering only biological factors typically lose half the available marks.
Topic 10 — Common Mistakes
- 🧬Listing measles or smallpox as a 9700 diseaseMeasles and smallpox were in older syllabuses but are NOT in 2025-2027. The four named diseases are cholera, malaria, TB, HIV/AIDS. Including measles or smallpox in a 9700 answer wastes time and may not be credited.
- ⚠Describing the malarial parasite life cycle in detail9700 (2025-2027) explicitly does NOT require knowledge of the Plasmodium life cycle. Sporozoites, merozoites, gametocytes, liver stages, blood stages — all unnecessary detail. Stick to: female Anopheles mosquito as vector; biological/social/economic factors for control.
- ⚗Saying that HIV can be transmitted by mosquitoes or casual contactBoth are wrong. HIV is not transmitted by mosquitoes (the virus does not survive or replicate in the mosquito, and mosquitoes do not inject blood from previous meals). Casual contact — handshakes, hugs, sharing food, sneezing, toilet seats — does not transmit HIV. Be careful with this in answers; the misconception fuels stigma.
- 🨍Confusing "antibiotic" with "antiviral"Antibiotics target bacteria. Antivirals (a separate class) target viruses. Saying "antibiotics treat HIV" is wrong — HIV needs antiretroviral therapy (ART). Get the terminology right.
- 🧤Saying "antibiotics cause mutations that produce resistance"Wrong. Mutations occur randomly, before and independent of antibiotic exposure. The antibiotic acts as a selection pressure: it favours pre-existing resistant variants, not creates them. This is one of the most-marked errors on Topic 10 questions.
- 🏥Saying viruses "have a small cell wall"Viruses are not cells and have no cell wall. They have a protein coat (capsid) and may have a lipid envelope, but no peptidoglycan, no plasma membrane in the bacterial sense. This is precisely why antibiotics don't work on them.
- 🧸Saying penicillin kills all bacteria including those that aren't growingPenicillin only kills actively growing bacteria, because it inhibits cell wall synthesis — cells must be making new wall to be affected. Dormant or stationary bacteria with complete walls are temporarily unaffected. This explains why incomplete antibiotic courses (stopping when symptoms ease) leave dormant bacteria alive to reactivate later.
- 🤯Stating that antibiotic resistance only spreads through reproductionResistance also spreads horizontally via plasmids — one bacterium passing a plasmid containing the resistance gene to an unrelated bacterium, even of a different species. Horizontal gene transfer is a major reason resistance spreads so fast.
- 🌏Discussing only biological factors when asked about prevention/controlThe syllabus LO requires biological + social + economic factors. An answer about malaria or HIV that mentions only drugs and vaccines (biology) misses the social and economic dimensions and typically loses half the marks. Always think across all three categories.
- 🏭Generalising about specific countries or populationsAvoid statements like "TB is common in [country]" or naming specific populations. The exam-relevant facts are biological (transmission mechanism) and structural (overcrowding, healthcare access, sanitation infrastructure). Frame answers around conditions that affect transmission, not on cultural or geographic stereotypes.
- 🤗Confusing HIV with AIDSHIV is the virus. AIDS is the late-stage clinical condition that develops when HIV has destroyed enough T-helper cells that opportunistic infections take hold. A person can be HIV-positive for many years without having AIDS, especially with antiretroviral therapy. They are not interchangeable terms.
Topic 10 is a knowledge-and-application topic rather than a mechanism-heavy topic. The four diseases must be matched to their pathogen, transmission, and control measures with high precision. Topic 11 (immunity) follows directly, and the immune-cell context for HIV/AIDS becomes much clearer there. Synoptic links: bacterial cell biology (Topic 1), water and sanitation (Topic 8 hydrostatic balance), evolution and natural selection (Topic 17 at A2 — antibiotic resistance is the canonical evolution example). Highest-yield items: the four pathogens with their full Latin names, the transmission route for each disease, the biological/social/economic structure for prevention answers, the penicillin mechanism with peptidoglycan and osmotic lysis, why antibiotics don't affect viruses, and antibiotic resistance via mutation + selection. Discuss-style questions reward balanced coverage; describe-style questions reward precision and correct terminology.