AS Level Biology chapter 10: Infectious diseases MCQs with answers
By Team Megalecture · Updated 4 Nov 2026
22 exam-style questions on Infectious diseases, each with the answer and a short explanation. Ten of them are below to try. The chapter test draws ten fresh questions each time and is marked the moment you finish, with your score by topic.
Try each one, then open the answer. These ten stay the same; the test draws new ones every time.
1. How does penicillin kill bacteria?
A
it breaks down the bacterial DNA, so the bacterium cannot replicate and eventually dies
B
it prevents ATP synthesis in bacterial mitochondria, so the cell cannot make cell wall
C
it inhibits the enzymes that form cross-links in the cell wall, so the weakened wall bursts as water enters
D
it binds to the ribosomes of the bacterium and stops them making proteins, so the cell dies slowly
Show answer
Answer: C. Without cross-links, new wall is weak; water enters by osmosis and the cell bursts (osmotic lysis).
2. How is malaria transmitted?
A
in droplets when an infected person coughs
B
by the bite of a female Anopheles mosquito
C
in water contaminated with the faeces of infected people
D
by sexual contact with an infected person, as in HIV
Show answer
Answer: B. Plasmodium is passed between people by the bite of an infected female Anopheles mosquito, which acts as the vector.
3. Why does prescribing antibiotics for viral infections such as colds increase antibiotic resistance?
A
the antibiotics do nothing to the virus but do select for resistant bacteria in the body
B
the cold virus infects the bacteria and passes its resistance genes on to them
C
the antibiotics weaken the patient's immune system, so the resistant bacteria multiply unchecked
D
the antibiotics change the viruses so that they pass resistance genes to the patient's bacteria
Show answer
Answer: A. The antibiotic does nothing to the virus, but the body's bacteria are exposed to it, so any resistant ones are favoured.
4. The same antibiotic-resistance gene is found in two unrelated species of bacteria in a hospital ward. What is the most likely explanation?
A
the gene is present in every bacterium and is switched on only when the antibiotic is used
B
the antibiotic caused the same mutation in both species at once
C
the gene was passed between the two species by meiosis
D
a plasmid carrying the gene was passed from one species to the other
Show answer
Answer: D. Resistance genes are often carried on plasmids, which bacteria can transfer to other cells, even of a different species. Mutations arise by chance, not because of the antibiotic, and bacteria do not undergo meiosis.
5. After floods in a district, 40 of 200 households drinking from well A (which has sewage seeping into it) have a case of cholera, compared with 2 of 200 households drinking from well B (which is sealed and clean). Which conclusion is best supported by these data?
A
cholera is spread by airborne droplets between people in the same district
B
cholera is spread by drinking water contaminated with faeces
C
cholera is spread by mosquitoes that breed in the flood water
D
cholera is spread by the direct exchange of blood between households
Show answer
Answer: B. The attack rate is 20% for well A households and 1% for well B, so illness follows the contaminated water supply. This fits faecal–oral transmission of Vibrio cholerae. Nothing in the data points to droplets, vectors or blood.
6. Why does penicillin not harm human cells?
A
human cell membranes are impermeable to penicillin, so it cannot enter any human cell
B
human cells are too large for the antibiotic to enter
C
human cells divide too slowly for penicillin to have any effect
D
human cells have no peptidoglycan cell wall, which is the target of penicillin
Show answer
Answer: D. The target, enzymes making peptidoglycan cross-links, does not exist in human cells, which have no cell wall.
7. Which pathogens cause tuberculosis (TB)?
A
Mycobacterium tuberculosis and Mycobacterium bovis
B
Plasmodium malariae and Mycobacterium bovis
C
Mycobacterium tuberculosis and Mycobacterium leprae, a related bacterium
D
Mycobacterium tuberculosis and Vibrio cholerae
Show answer
Answer: A. TB is caused by the bacteria Mycobacterium tuberculosis and Mycobacterium bovis. HIV weakens the immune system, making TB more likely, but does not cause it.
8. A village uses insecticide-treated bed nets and drains standing water near homes. Malaria cases fall from 320 per 1000 people in one year to 80 per 1000 people in the next. By what percentage have cases fallen?
A
25%
B
75%
C
240%
D
300%
Show answer
Answer: B. The fall is 320 − 80 = 240 cases per 1000, and 240 ÷ 320 × 100 = 75%. Both measures target the mosquito vector, cutting the chance of transmission. 25% is the proportion of cases that remain.
9. A city of 500 000 people reports 1250 new cases of TB in one year. What is the incidence of TB per 100 000 people?
A
25
B
250
C
400
D
2500
Show answer
Answer: B. Incidence per 100 000 = 1250 ÷ 500 000 × 100 000 = 250. Dividing 500 000 by 1250 gives 400, which is the number of people per case, not the incidence. Overcrowded housing and poverty raise TB incidence because droplets spread easily.
10. Which type of pathogen causes malaria?
A
a bacterium
B
a virus
C
a fungus
D
a protoctist
Show answer
Answer: D. Malaria is caused by Plasmodium species, which are protoctists; the mosquito is only the vector.
22, each with the answer and a short explanation, across 2 topics: Infectious diseases, Antibiotics. Every chapter test draws ten of them, so you can take it many times.
Is the AS Level Biology chapter 10 test free?
Yes. The test, the answers and the explanations are free and need no sign-up. Sign in only if you want your scores kept across devices.
How is the chapter test marked?
The moment you press Finish: your score, the right answer and an explanation for every question, and the ones you got wrong saved in My mistakes to retry.
