Question 1
Paper 4 short-answer style1 markName one sexually transmitted infection. (1 mark)
Model answer
HIV (which can lead to AIDS).
Why this scores
One mark for HIV (or another named STI such as gonorrhoea).
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Detailed notes on Reproduction for Cambridge IGCSE Biology, covering key concepts, explanations, examples, and exam-focused revision points.
Diseases passed by sex. Cambridge focus on HIV/AIDS — a virus that destroys the immune system. Prevention is straightforward; treatment is lifelong.
Mapped to the Cambridge IGCSE 0610 syllabus (2026-2028).
Diseases caught from sexual contact. Some bacterial (curable); some viral (manageable).
STI = sexually transmitted infection. Sometimes called STD (sexually transmitted disease).
Pathogens involved:
Transmission routes:
NOT transmitted by:
Worked qualitative. Why are STIs called 'silent' diseases?
Cambridge tip. Cambridge mainly tests HIV. Know the basic facts about it.
HIV = the virus; AIDS = the late stage. HIV infects and destroys lymphocytes → fewer antibodies produced → body cannot fight off infections → leads to AIDS. Manageable but not curable.
HIV = Human Immunodeficiency Virus. A retrovirus.
What HIV does inside the body:
The progression to AIDS:
| Stage | What's happening |
|---|---|
| HIV infection | Virus present; few or no symptoms |
| Asymptomatic phase | Years, even decades; lymphocytes slowly declining |
| Symptomatic HIV | Frequent minor infections; weight loss; tiredness |
| AIDS | Lymphocyte count very low; opportunistic infections + cancers; without treatment, death within 1-3 years |
Opportunistic infections.
Worked qualitative. Why is the HIV pandemic harder to fight than something like measles?
Cambridge tip. Memorise the mark-scheme chain: HIV infects and destroys LYMPHOCYTES → fewer antibodies produced → body cannot fight off infections → leads to AIDS. Saying 'attacks immune system' is too vague.
Condoms, clean needles, blood screening, treatment-as-prevention. Education matters.
1. Condoms.
2. Don't share needles.
3. Blood screening.
4. Antiretroviral therapy (ART).
5. PrEP (pre-exposure prophylaxis).
6. Mother-to-baby prevention.
7. Education and testing.
Worked qualitative. Why is breastfeeding controversial for HIV+ mothers?
Cambridge tip. When asked HOW to limit HIV, give 4-5 strategies. Cambridge marks each.
Verbatim phrases and definitions Cambridge mark schemes credit.
STIs appear Paper 4 (3-5 marks: HIV mechanism + prevention). Examiner reports flag students confusing HIV with AIDS, claiming HIV is curable, and missing 'lymphocyte' as the target.
Sources: Cambridge IGCSE Biology 0610 syllabus 2026-2028 (16.6); 0610/42 Oct/Nov 2024 — Q16 (HIV/AIDS); 0610 Examiner Reports 2022-2024. Last reviewed 2026-05-07.
Step-by-step solutions to past-paper-style questions on sexually transmitted infections, written exactly the way a tutor would explain them at the board.
Question
State what is meant by a sexually transmitted infection (STI) and give an example.
Step-by-step solution
Step 1
A sexually transmitted infection (STI) is an infection that is transmitted (passed on) through sexual contact.
Step 2
It is caused by a pathogen passed from one person to another during sexual activity.
Step 3
An example is HIV (human immunodeficiency virus), which can lead to AIDS.
Answer
An STI is an infection passed on through sexual contact (caused by a pathogen). An example is HIV.
Question
State three ways in which HIV can be passed from one person to another.
Step-by-step solution
Step 1
Through unprotected sexual contact with an infected person.
Step 2
Through infected blood, for example when drug users share needles.
Step 3
From an infected mother to her baby — across the placenta, during birth, or through breast milk.
Answer
By unprotected sex, by infected blood (e.g. sharing needles), and from an infected mother to her baby.
Examiner tip
HIV spreads through body fluids (blood, semen, vaginal fluids, breast milk) — not through casual contact.
Question
Explain how HIV affects the body and leads to AIDS.
Step-by-step solution
Step 1
HIV is a virus that infects and destroys lymphocytes — a type of white blood cell that is part of the immune system.
Step 2
Over time, the number of these white blood cells falls, so the immune system becomes weaker.
Step 3
The body becomes unable to fight off other infections that a healthy person could resist.
Step 4
This advanced stage of HIV infection, when the immune system has been badly damaged, is called AIDS.
Answer
HIV destroys lymphocytes (white blood cells), weakening the immune system, so the person cannot fight off other infections — the advanced stage is AIDS.
Question
Describe four ways the transmission of HIV can be reduced.
Step-by-step solution
Step 1
Use a condom during sex, as a barrier to body fluids.
Step 2
Do not share needles (for example among drug users).
Step 3
Screen donated blood so that no infected blood is used in transfusions.
Step 4
Education and testing so people know how it spreads and their status; treating infected mothers reduces transmission to the baby.
Answer
Use condoms, do not share needles, screen donated blood, and educate/test people (and treat infected mothers).
Question
Explain why a person with AIDS often becomes seriously ill from infections that a healthy person could fight off easily.
Step-by-step solution
Step 1
HIV destroys the lymphocytes, the white blood cells that make antibodies and help fight pathogens.
Step 2
As more lymphocytes are destroyed, the immune system becomes too weak to respond properly to pathogens.
Step 3
So when a pathogen enters, the body cannot make enough antibodies to destroy it, and the infection takes hold.
Step 4
Infections that a healthy immune system would easily control therefore become serious or even fatal in a person with AIDS.
Answer
HIV destroys the lymphocytes, so the immune system cannot make enough antibodies to fight pathogens. Infections a healthy person would resist can then become serious or fatal.
Examiner tip
Link the destruction of lymphocytes to the inability to make antibodies and so fight other infections.
Question
A health programme provides free condoms, clean needles, blood screening and education about HIV. Explain how each measure helps to reduce the spread of HIV in the population.
Step-by-step solution
Step 1
Condoms act as a barrier during sex, preventing the exchange of body fluids that carry HIV.
Step 2
Clean needles stop HIV being passed in blood when drug users no longer share needles.
Step 3
Blood screening ensures that only HIV-free blood is used in transfusions, removing one route of transmission.
Step 4
Education helps people understand how HIV spreads, so they can avoid risky behaviour and get tested — together these break the different routes of transmission and slow the spread.
Answer
Each measure blocks a route of transmission: condoms (sexual contact), clean needles (infected blood), screening (transfusions), education (risky behaviour). Together they reduce HIV spread.
Examiner tip
Match each measure to the transmission route it interrupts — that reasoning earns the marks.
High-scoring sample answers for sexually transmitted infections on the Cambridge IGCSE 0610 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.
Name one sexually transmitted infection. (1 mark)
Model answer
HIV (which can lead to AIDS).
Why this scores
One mark for HIV (or another named STI such as gonorrhoea).
State two ways in which HIV can be transmitted from one person to another. (2 marks)
Model answer
Through unprotected sexual contact with an infected person, and through infected blood (for example when needles are shared). (From an infected mother to her baby is also accepted.)
Why this scores
Two distinct routes: sexual contact; infected blood/needles; mother to baby.
Explain how HIV affects the immune system. (3 marks)
Model answer
HIV infects and destroys lymphocytes, which are a type of white blood cell that defends the body. As the number of these cells falls, the immune system becomes weaker and the body becomes less able to fight off other infections.
Why this scores
Three marks: destroys lymphocytes/white blood cells; immune system weakened; less able to fight infections.
Describe how the transmission of HIV can be reduced. (4 marks)
Model answer
Using a condom during sex provides a barrier that prevents the exchange of body fluids. Not sharing needles prevents HIV being passed in blood between drug users. Screening donated blood ensures that no infected blood is used in transfusions. Educating people about how HIV spreads, and providing testing, helps people avoid risky behaviour and know their status (and treating infected pregnant women reduces transmission to the baby).
Why this scores
Four marks from: condoms; not sharing needles; screening blood; education/testing; treating infected mothers.
Explain why a person with AIDS may die from an infection that a healthy person could easily fight off. (5 marks)
Model answer
HIV infects and destroys the lymphocytes, the white blood cells that produce antibodies to fight pathogens. Over time, so many are destroyed that the immune system is severely weakened — this stage is AIDS. When a pathogen enters the body, there are too few lymphocytes to produce enough antibodies, so the body cannot destroy the pathogen effectively. The infection therefore spreads and becomes serious, and an infection that a healthy immune system would easily control can prove fatal in a person with AIDS.
Why this scores
Five marks: HIV destroys lymphocytes; these normally make antibodies; immune system severely weakened (AIDS); too few cells to make antibodies; cannot fight the pathogen, so even minor infections become fatal.
Describe how HIV is transmitted and explain how its spread through a population can be controlled. (6 marks)
Model answer
HIV is transmitted in body fluids. It is passed on through unprotected sexual contact with an infected person, through infected blood — for example when drug users share needles or, rarely, in a blood transfusion — and from an infected mother to her baby across the placenta, during birth, or in breast milk. Its spread can be controlled by interrupting these routes. Using condoms during sex provides a barrier to body fluids; not sharing needles prevents transmission in blood; screening all donated blood ensures infected blood is not used in transfusions; and treating infected pregnant women reduces the chance of passing HIV to the baby. Educating people about how HIV spreads, and providing testing so people know whether they are infected, helps them avoid risky behaviour. Because HIV is only spread through these specific routes, blocking each one greatly reduces the spread of the virus through the population.
Why this scores
Up to 6 marks: transmission routes (sexual contact, infected blood/needles/transfusions, mother to baby); control measures matched to routes (condoms, no needle sharing, blood screening, treating mothers, education/testing). Reward linking each control to the route it blocks.
Definitions to memorise and the exact keywords mark schemes credit for sexually transmitted infections answers — sharpened from recent examiner reports for the 2026 0610 sitting.
An infection transmitted through sexual contact, caused by a pathogen passed from one person to another.
Human immunodeficiency virus. It infects and destroys lymphocytes (white blood cells), weakening the immune system.
The advanced stage of HIV infection, when the immune system is so weakened that the body cannot fight off other infections.
The traps other students keep falling into on sexually transmitted infections questions — taken from recent Cambridge IGCSE 0610 examiner reports and mark schemes — and how to avoid them.
Why it happens
They are closely related.
How to avoid it
HIV is the VIRUS. AIDS is the advanced stage of infection it causes once enough immune cells are destroyed. A person can be HIV-positive for years before developing AIDS, especially with treatment.
Why it happens
Misinformation and stigma.
How to avoid it
HIV spreads only through body fluids (blood, semen, vaginal fluids, breast milk). It is not spread by hugging, sharing cups, or insect bites.
Why it happens
Students know HIV is in the blood.
How to avoid it
HIV infects and destroys LYMPHOCYTES (white blood cells of the immune system), not red blood cells.
The things students keep getting wrong in this sub-topic, answered.