Cambridge International AS & A Level Biology 9700: Infectious diseases (by subtopic) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover topic 10 of the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027: sub-topic 10.1 (Infectious diseases) and sub-topic 10.2 (Antibiotics). This is AS Level content, the basis of Paper 1 and Paper 2 and required knowledge for Paper 4. Calculators are allowed. Each question is tagged with the outcome it tests.

Course links: Cambridge A Level Biology hub, printable 9700 checklist and the free 9700 AS diagnostic.

Questions

1. (10.1.1) State what is meant by the term transmissible, and explain why lung cancer is not an infectious disease. [2]

2. (10.1.2) A student wrote these four statements. Each one contains an error. Rewrite each statement correctly.

(i) Malaria is caused by the bacterium Plasmodium vivax. (ii) Tuberculosis is caused by one species of bacterium, Mycobacterium tuberculosis. (iii) AIDS is a virus that attacks T-helper lymphocytes. (iv) Cholera is caused by the protoctist Vibrio cholerae. [4]

3. (10.1.3) A town with cases of cholera and TB tells every household to boil drinking water.

(a) Explain why boiling water reduces cholera cases but has no effect on TB caused by Mycobacterium tuberculosis. [3] (b) Name a food in which Mycobacterium bovis can reach humans. [1]

4. (10.1.3, 10.1.4) A pregnant woman tests positive for HIV.

(a) Describe three ways HIV could pass from her to her child. [3] (b) Suggest two ways the risk of passing HIV to the child can be reduced. [2]

5. (10.1.3, 10.1.4) Female Anopheles mosquitoes from two villages were exposed to the insecticide used on bed nets. In village P, 24 of 160 survived. In village Q, where treated nets had been used for much longer, 99 of 220 survived.

(a) Calculate the percentage of mosquitoes that survived in each village. [2] (b) Explain why a higher percentage survived in village Q. [3] (c) Discuss two other measures to reduce malaria in village Q, giving one factor that could limit each. [4] (d) Explain how a female Anopheles mosquito passes Plasmodium from one person to another. [3]

6. (10.1.4) After an earthquake, several thousand people move into a temporary camp. Discuss the biological, social and economic factors that health workers need to consider to prevent a cholera outbreak in the camp. [6]

7. (10.1.4, 10.2.2) In one year, 400 TB patients in a district started a six-month drug course and 340 completed it. The next year, health workers watched each patient take every dose: 450 started and 423 completed.

(a) Calculate the percentage of patients who completed the course in each year. [2] (b) Suggest two social or economic reasons why some patients do not complete a six-month course. [2] (c) Explain how a higher completion rate reduces the transmission of TB in the district. [2] (d) Some patients carried strains of M. tuberculosis resistant to two of the drugs used. Discuss the consequences of this for the patients and for the district. [3]

8. (10.1.4) This question is about controlling HIV.

(a) Explain why it has been difficult to develop an effective vaccine against HIV. [3] (b) Outline how widespread testing combined with anti-retroviral therapy (ART) reduces the spread of HIV in a population. [2]

9. (10.2.1) Penicillin was added to two flasks of the same growing culture of a rod-shaped bacterium. Flask X contained dilute nutrient broth. Flask Y contained the same broth with a high concentration of sucrose. After three hours, the cells in flask X had burst. The cells in flask Y were intact but had become spherical. Explain these results. [4]

10. (10.2.1) Some antibiotics act by binding to bacterial 70S ribosomes. Explain why such an antibiotic has no effect on a virus, even when the virus is replicating inside a human cell. [3]

11. (10.2.2) A hospital recorded antibiotic prescriptions and the percentage of samples of one bacterial species resistant to a common antibiotic. A policy to cut unnecessary prescriptions began in January 2021.

Year	Prescriptions per 1000 patient-days	Samples resistant / %
2018	620	18
2019	640	22
2020	655	27
2021	480	28
2022	450	26
2023	430	24

(a) Calculate the percentage decrease in prescriptions per 1000 patient-days from 2020 to 2023. [2] (b) Suggest why the percentage of resistant samples did not fall straight away in 2021. [2] (c) Describe three consequences for patients if resistance had kept rising. [3] (d) Other than reducing prescriptions, describe three steps the hospital could take to reduce the impact of antibiotic resistance. [3]

Answers

1. Transmissible means the pathogen can be passed from one host to another [1]. Lung cancer is not caused by a pathogen, so it cannot be passed between people [1]. Examiner insight: “Contagious” or “spreads” alone is too vague for the first mark; the answer must say it passes from one person (host) to another.

2. (i) Malaria is caused by the protoctist Plasmodium vivax [1]. (ii) TB is caused by two species: M. tuberculosis and M. bovis [1]. (iii) HIV is the virus; AIDS is the disease/syndrome it causes [1]. (iv) Cholera is caused by the bacterium Vibrio cholerae [1]. Examiner insight: Each mark needs the wrong word replaced; write the genus with a capital, the species in lower case, and underline both by hand.

3. (a) Cholera is spread by the faecal–oral route: V. cholerae from faeces contaminates drinking water [1]. Boiling kills the bacteria in the water, so they are not swallowed [1]. TB caused by M. tuberculosis spreads in airborne droplets from coughs and sneezes, not in water, so boiling does not break its route [1]. (b) Unpasteurised milk (accept undercooked meat from infected cattle) [1]. Examiner insight: The TB mark needs the route stated (droplets in air); “TB is not in water” alone does not explain anything.

4. (a) Across the placenta during pregnancy [1]; through blood or body fluids during birth [1]; in breast milk [1]. (b) The mother takes ART during pregnancy and breastfeeding so her viral load is low [1]; the baby is given anti-retroviral drugs after birth [1]. (Also accept formula feeding where safe, clean water is available.) Examiner insight: “Through blood” with no stage named is too vague; each mark in (a) needs a different stage: pregnancy, birth or breastfeeding.

5. (a) P: 24 ÷ 160 × 100 = 15.0% [1]; Q: 99 ÷ 220 × 100 = 45.0% [1]. (b) Some mosquitoes already had a mutation giving resistance [1]. The insecticide was a selection pressure: susceptible mosquitoes died, resistant ones survived and reproduced [1]. The allele passed to offspring; longer net use in Q meant more generations of selection [1]. (c) Drain or cover standing water where larvae develop [1]; but mosquitoes breed in any small pool (biological) [1]. Treat cases quickly with antimalarial drugs [1]; but Plasmodium can be drug-resistant, or drugs cost money (economic) [1]. (d) Feeding on an infected person’s blood, she takes in Plasmodium [1]; the parasite develops in the mosquito and reaches its salivary glands [1]; when she bites another person, saliva containing Plasmodium enters their blood [1]. Examiner insight: In (c) each measure needs a limiting factor; four measures with no factors earn at most two marks.

6. Biological: latrines must be kept away from water supplies, as faeces carry V. cholerae [1]; symptomless carriers still shed bacteria [1]; patients need oral rehydration therapy quickly because the toxin causes severe watery diarrhoea [1]. Social: overcrowding makes contamination of shared water and food more likely [1]; hand-washing and food-hygiene education is needed [1]. Economic: chlorinated water, soap and sanitation cost money that may not be available quickly [1]. Examiner insight: Full marks need points from all three categories, not biology alone.

7. (a) First year: 340 ÷ 400 × 100 = 85.0% [1]; next year: 423 ÷ 450 × 100 = 94.0% [1]. (b) Patients feel better after a few weeks and stop [1]; they cannot afford travel to the clinic or time off work [1]. (Also accept stigma making them hide the illness.) (c) Patients who finish are cured and no longer release bacteria in droplets [1], so fewer people breathe in M. tuberculosis, especially in crowded homes [1]. (d) The usual drugs do not cure these patients, so they need other drugs, often for longer [1]; they stay infectious longer and pass the resistant strain to contacts in droplets [1]; costs to the district and deaths may rise [1]. Examiner insight: (d) names two viewpoints; three points about the patient with nothing about the district cannot earn the second mark.

8. (a) HIV mutates rapidly, so its antigens change and antibodies to one form may not bind the next [1]; there are many strains [1]; HIV infects T-helper cells, which coordinate the immune response, and hides inside host cells [1]. (b) Testing finds infected people who did not know [1]; ART reduces the viral load, so they are much less likely to pass HIV on [1]. Examiner insight: “HIV mutates” earns credit only when linked to changed antigens and antibody or memory-cell specificity.

9. Penicillin inhibits the enzymes that form cross-links in new peptidoglycan, so the walls of growing cells are weakened [1]. In X the broth has a higher water potential than the cytoplasm, so water enters by osmosis and the weak wall bursts: osmotic lysis [1]. In Y the sucrose lowers the water potential of the medium, so there is no net water entry [1]. The cells become spherical because the wall gives the cell its shape [1]. Examiner insight: Use “water potential”, not “water concentration”; the flask Y mark needs “no net entry of water”, not just “sucrose stops lysis”.

10. A virus has no ribosomes of its own (no cell structure or metabolism) [1]. It makes its proteins using the host cell’s ribosomes, which in the cytoplasm are 80S, not 70S [1], so the antibiotic has no target and does not stop viral replication [1]. Examiner insight: “Viruses are not alive” scores nothing; the mark needs the missing target (no wall, no ribosomes, no metabolism) named.

11. (a) 655 − 430 = 225 [1]; 225 ÷ 655 × 100 = 34.4% [1]. (b) Resistant bacteria were still present and had already spread [1]; less antibiotic lowers the selection pressure but does not kill resistant bacteria, so they decline only slowly [1]. (c) Infections take longer to treat or cannot be treated with usual drugs [1]; more deaths from once-curable infections [1]; surgery and cancer treatment become riskier [1]. (d) Hand-washing and isolating infected patients so resistant bacteria spread less [1]; testing the bacterium before prescribing and using a narrow-spectrum antibiotic [1]; rotating the antibiotics used, or making sure patients finish the full course [1]. Examiner insight: In (a) the divisor must be the original value (655); dividing 225 by 430 gives 52.3% and earns only the subtraction mark.

Where marks are usually lost

• Calling Plasmodium a bacterium or the mosquito the pathogen.
• Writing “AIDS virus”: HIV is the virus, AIDS is the condition.
• Forgetting M. bovis when asked for both TB pathogens.
• Linking cholera to the air or TB to water.
• Saying insecticide or antibiotic “causes” the resistance mutation.

Next steps

• Recap the tables: infectious diseases revision notes
• Relearn a weak outcome: infectious diseases study guide
• More questions: infectious diseases practice questions
• Course overview: Cambridge A Level Biology hub
• Track every outcome: printable 9700 checklist
• 9700 AS Level diagnostic, or all free 10-minute diagnostics
• Book a free trial class

Official syllabus

Cambridge International, Cambridge International AS & A Level Biology 9700 syllabus for 2025, 2026 and 2027 (Version 1, published September 2022), Cambridge University Press & Assessment. Topic 10, Infectious diseases: sections 10.1 and 10.2.