Cambridge International AS & A Level Biology 9700: Infectious diseases – Study Guide

This study guide teaches topic 10 Infectious diseases of the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027. It covers section 10.1 Infectious diseases (learning outcomes 1–4) and section 10.2 Antibiotics (learning outcomes 1–2). This is AS Level content, so it is examined on Paper 1 (Multiple Choice) and Paper 2 (AS Level Structured Questions). The syllabus also states that Paper 4 needs knowledge of AS Level content, so you may use it again at A Level. There is no Core/Supplement split in 9700: every outcome here applies to every candidate.

Useful links: Cambridge AS & A Level Biology hub, printable 9700 checklist, infectious diseases revision notes, infectious diseases practice questions and the free 9700 AS diagnostic. Topic 11 follows on directly: see the immunity study guide.

What this topic covers

Syllabus section	What you must be able to do
10.1.1	State that infectious diseases are caused by pathogens and are transmissible
10.1.2	State the name and type of pathogen for cholera, malaria, TB and HIV/AIDS
10.1.3	Explain how cholera, malaria, TB and HIV are transmitted
10.1.4	Discuss the biological, social and economic factors in the prevention and control of these four diseases (the malarial life cycle is not expected)
10.2.1	Outline how penicillin acts on bacteria and why antibiotics do not affect viruses
10.2.2	Discuss the consequences of antibiotic resistance and the steps to reduce its impact

10.1 Infectious diseases

What makes a disease infectious

An infectious disease is caused by a pathogen: an organism (or a virus) that causes disease. The key word in the syllabus is transmissible: the pathogen can pass from one host to another. Non-infectious diseases, such as lung cancer, cannot be caught from another person.

The four named diseases and their pathogens

Learn these exactly: capital letter for the genus, lower case for the species.

Disease	Pathogen	Type of pathogen
Cholera	Vibrio cholerae	Bacterium
Malaria	Plasmodium falciparum, P. malariae, P. ovale, P. vivax	Protoctist
Tuberculosis (TB)	Mycobacterium tuberculosis and Mycobacterium bovis	Bacteria
HIV/AIDS	Human immunodeficiency virus (HIV)	Virus

Only the two bacterial diseases can be treated with antibiotics.

How each disease is transmitted

Cholera is spread by the faecal–oral route. The bacteria leave an infected person in faeces. If sewage gets into drinking water, or if food is washed in contaminated water or handled with unwashed hands, the next person swallows the bacteria. V. cholerae multiplies in the small intestine and releases a toxin. The toxin causes the intestinal lining to lose chloride ions and water into the gut, producing severe watery diarrhoea and rapid dehydration.

Malaria is spread by a vector, the female Anopheles mosquito. When an infected mosquito takes a blood meal, Plasmodium passes from its salivary glands into the person’s blood. A mosquito that bites an infected person picks up the parasite in the blood it takes. Malaria can also pass in blood transfusions, by sharing needles and from mother to fetus across the placenta. You do not need the stages of the parasite’s life cycle.

TB caused by M. tuberculosis is spread in the air. A person with active TB in the lungs coughs, sneezes or talks, releasing droplets containing the bacteria. Other people breathe them in. M. bovis infects cattle and passes to humans mainly in unpasteurised milk or undercooked meat from infected animals.

HIV is spread by the exchange of body fluids: unprotected sexual intercourse; blood, through sharing needles or transfusion of unscreened blood; and from mother to child across the placenta, during birth or in breast milk. HIV infects T-helper lymphocytes, so over time the immune system fails and the person develops AIDS, becoming vulnerable to opportunistic infections.

Prevention and control: the three kinds of factor

Outcome 10.1.4 says “discuss”: say what a control method does, then give a biological, social or economic factor that helps or limits it.

Disease	Biological factors	Social factors	Economic factors
Cholera	Carriers can have no symptoms but still shed bacteria; oral rehydration therapy treats dehydration; vaccines give limited, short-term protection	Overcrowding after floods, earthquakes or war; refugee camps with poor sanitation; hand-washing habits	Sewage treatment and piped, chlorinated water are expensive to build and maintain
Malaria	Mosquitoes become resistant to insecticides; Plasmodium becomes resistant to antimalarial drugs; the parasite lives inside liver cells and red blood cells, hidden from the immune system, which makes vaccine development hard; mosquitoes breed in any standing water	Sleeping under nets needs regular use; people may live far from clinics; war and migration move people into malarial areas	Insecticide-treated nets, spraying, drugs and diagnosis all cost money; many affected countries have limited health budgets
TB	Treatment needs several antibiotics for many months; drug-resistant strains exist; HIV weakens immunity and allows latent TB to become active; the BCG vaccine does not protect everyone	Overcrowded, poorly ventilated housing; people stop treatment when they feel better; stigma delays testing	Long courses of drugs and supervised treatment are costly; pasteurisation and testing of cattle need money
HIV	No vaccine; HIV mutates quickly, so its antigens change; it hides inside T-helper cells; anti-retroviral therapy (ART) lowers viral load but does not cure	Stigma discourages testing; education about condoms and safe sex; needle-exchange schemes; screening of donated blood	ART must be taken for life, which is expensive; testing and education programmes need funding

Control measures to name, disease by disease:

• Cholera: treat sewage; supply chlorinated or boiled water; good food hygiene; hand-washing; oral rehydration therapy for patients.
• Malaria: insecticide-treated bed nets; spraying insecticide inside houses; draining or covering standing water, or adding fish that eat mosquito larvae; antimalarial drugs; diagnosing and treating people quickly so fewer mosquitoes pick up the parasite.
• TB: BCG vaccination; contact tracing and testing; long, supervised drug treatment (health workers watch patients take each dose); isolating infectious patients; pasteurising milk and testing cattle for M. bovis.
• HIV: condoms; testing and contact tracing; screening blood for transfusion; needle-exchange programmes; ART for infected people, which also lowers the chance they pass HIV on; ART for pregnant women to reduce transmission to the baby.

Worked example: evaluating a control measure

A village introduces insecticide-treated bed nets for every household. In the year before, there were 480 cases of malaria. In the year after, there were 312.

(a) Calculate the percentage decrease in cases.

decrease            = 480 − 312 = 168 cases
percentage decrease = (168 ÷ 480) × 100 = 35.0%

(b) Suggest why cases did not fall to zero.

1. Mosquitoes bite outdoors or in the evening, before people go to bed, so nets do not stop every bite.
2. Some nets may be torn, not used every night, or no longer treated with active insecticide.
3. Some mosquitoes may be resistant to the insecticide.
4. People already infected before the nets arrived still count as cases, and people who travel may be bitten elsewhere.

Each suggestion links a factor to how the net works, which is what “discuss” rewards.

10.2 Antibiotics

How penicillin acts on bacteria

A bacterial cell wall is made of peptidoglycan: long polysaccharide chains held together by short peptide cross-links. The wall stops the cell bursting when water enters by osmosis.

As a bacterium grows, enzymes make small breaks in the wall so new material can be added. Other enzymes, transpeptidases, then form new cross-links. Penicillin inhibits the enzymes that form the cross-links. The bacterium keeps making breaks but cannot seal them, so the wall becomes weaker. Water enters by osmosis, the cell swells and it bursts (osmotic lysis).

Two consequences follow:

• Penicillin works only on bacteria that are growing and making new cell wall.
• Human cells have no cell wall, so penicillin does not harm them.

Why antibiotics do not affect viruses

Antibiotics work by interfering with structures or processes found in bacteria: the peptidoglycan cell wall, bacterial (70S) ribosomes and bacterial enzymes. A virus has none of these: it is nucleic acid in a protein coat, with no metabolism of its own, and it replicates using the host cell’s enzymes and ribosomes. So there is no target for the antibiotic. This is why a doctor should not prescribe antibiotics for a cold, influenza or HIV.

Antibiotic resistance

How it arises. A random mutation in a bacterium may produce an allele that gives resistance, for example a gene coding for an enzyme that breaks down penicillin. The antibiotic does not cause the mutation.

How it spreads. When an antibiotic is used, it is a selection pressure. Susceptible bacteria die, while resistant ones survive and reproduce by binary fission, passing the allele to their offspring (vertical transmission). Resistance genes are often on plasmids, which can pass between bacteria, even between different species (horizontal transmission). Over time, a higher proportion of the population is resistant.

Consequences (the syllabus asks you to discuss these):

• Infections take longer to treat, or cannot be treated with the usual drugs.
• More people die from infections that used to be easy to cure.
• Patients stay longer in hospital, which raises costs and spreads infection.
• Newer antibiotics are more expensive and may have more side effects.
• Operations, cancer treatment and care of premature babies become riskier, because they depend on preventing infection.
• Diseases such as TB become much harder to control when strains are resistant to several drugs.

Steps to reduce the impact:

• Prescribe antibiotics only when needed, and never for viral infections.
• Use a narrow-spectrum antibiotic matched to the bacterium after testing.
• Take the full course exactly as prescribed; do not keep or share leftover tablets.
• Change the antibiotics used from time to time so no single one exerts selection pressure for long.
• Reduce antibiotic use in farming, especially to promote growth.
• Improve hospital hygiene (hand-washing, isolating infected patients) so resistant bacteria spread less.
• Fund research into new antibiotics.

Worked example: interpreting resistance data

A hospital tested samples of one bacterial species for resistance to an antibiotic.

Year	Samples tested	Samples resistant
2016	240	36
2020	260	57
2024	290	87

(a) Calculate the percentage of samples resistant in each year.

2016: (36 ÷ 240) × 100 = 15.0%
2020: (57 ÷ 260) × 100 = 21.9%
2024: (87 ÷ 290) × 100 = 30.0%

(b) Describe the trend. The percentage resistant rose every time it was measured, from 15.0% to 30.0%: it doubled between 2016 and 2024 (a rise of 15.0 percentage points).

(c) Explain the trend. Use of the antibiotic killed susceptible bacteria. Resistant bacteria, which already carried a mutation, survived and reproduced, so the resistance allele became more common in the population. Plasmids carrying the gene may also have passed to other bacteria.

Compare percentages, not raw numbers: the sample size changed each year.

Common errors

• Writing “Plasmodium is a bacterium” or “malaria is caused by a mosquito”. The mosquito is the vector; the pathogen is a protoctist.
• Misspelling pathogen names, or giving Vibrio cholerae for TB. Learn all seven species names in the table.
• Saying cholera is “airborne” or “spread by touch”. It is spread in contaminated water or food.
• Writing “HIV kills white blood cells” without saying which: it infects T-helper lymphocytes.
• Saying penicillin “makes holes in the wall” or “breaks down the wall”. It stops cross-links forming in new wall.
• Saying bacteria “become immune” or “get used to” antibiotics. Populations become resistant by mutation and selection.
• Listing control methods without linking them to biological, social or economic factors when the command word is “discuss”.

Where to go next

Condense this topic with the infectious diseases revision notes, then test yourself with the infectious diseases practice questions. The immune response to these pathogens is in the immunity study guide. Check your coverage against the 9700 checklist and try the 9700 AS diagnostic.

Official syllabus

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