What are Sexually Transmitted Infections (STIs) and how are they related to reproduction?

Sexually Transmitted Infections (STIs) are infections that are primarily spread through sexual contact. They are closely related to reproduction as they can affect the reproductive organs and overall reproductive health. Understanding STIs is crucial in the study of reproduction, especially in the context of the Cambridge IGCSE Biology curriculum, as it highlights the importance of safe sexual practices to prevent these infections.

What are the common symptoms of Sexually Transmitted Infections?

Common symptoms of STIs can vary depending on the infection but often include unusual discharge from the genitals, pain during urination, sores or bumps on the genitals, and itching or irritation. Some STIs may not show symptoms initially, which is why regular testing is important. In the context of Cambridge IGCSE Biology, recognizing these symptoms is essential for early diagnosis and treatment.

How can Sexually Transmitted Infections be prevented?

STIs can be prevented through the use of condoms during sexual activity, regular STI testing, and maintaining a mutually monogamous relationship where both partners have tested negative for STIs. Education on safe sexual practices, as emphasized in the Cambridge IGCSE Biology curriculum, is key to prevention.

What are the potential consequences of untreated Sexually Transmitted Infections?

Untreated STIs can lead to serious health issues, including infertility, chronic pain, and increased risk of acquiring other infections. Some STIs can also be passed from mother to child during childbirth. The Cambridge IGCSE Biology syllabus highlights the importance of understanding these consequences to promote awareness and encourage timely medical intervention.

Why is it important to study Sexually Transmitted Infections in the Cambridge IGCSE Biology curriculum?

Studying STIs in the Cambridge IGCSE Biology curriculum is important because it educates students on the biological aspects of these infections, their impact on human health, and the importance of prevention and treatment. This knowledge empowers students to make informed decisions about their reproductive health and contributes to public health awareness.

Why is "Using HIV and AIDS interchangeably." a common mistake in Sexually Transmitted Infections?

Why it happens: Both related. How to avoid it: HIV is the VIRUS. AIDS is the SYNDROME caused by the virus when it's destroyed enough immune cells. People can be HIV+ for years without having AIDS, especially with treatment.

Why is "Thinking HIV spreads through casual contact (sharing food, hugging)." a common mistake in Sexually Transmitted Infections?

Why it happens: Stigma + misinformation. How to avoid it: HIV needs DIRECT BLOOD/SEMEN/VAGINAL FLUID/BREAST MILK contact. Not via shared cups, hugs, kisses, mosquitoes, or sneezes.

Why is "Saying HIV is curable." a common mistake in Sexually Transmitted Infections?

Why it happens: Modern treatments are very effective. How to avoid it: HIV is currently INCURABLE — drugs SUPPRESS the virus to undetectable levels (manageable lifelong condition) but don't eliminate it. Major research goal.

ReproductionCambridge IGCSE Biology (0610)

Sexually Transmitted Infections

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Sexually Transmitted Infections — Cambridge IGCSE 0610 Biology Extended (2026)

Diseases passed by sex. Cambridge focus on HIV/AIDS — a virus that destroys the immune system. Prevention is straightforward; treatment is lifelong.

What you’ll learn

Mapped to the Cambridge IGCSE 0610 syllabus (2026-2028).

16.6 — Define an STI.
16.6 — Describe HIV/AIDS — pathogen, transmission, effects.
16.6 — Suggest ways to limit transmission.

What are STIs?

Diseases caught from sexual contact. Some bacterial (curable); some viral (manageable).

STI = sexually transmitted infection. Sometimes called STD (sexually transmitted disease).

Pathogens involved:

Bacteria: gonorrhoea, syphilis, chlamydia. Curable with antibiotics if caught early.
Viruses: HIV, herpes, hepatitis B, HPV (causes cervical cancer). Generally not curable, but manageable.
Other: pubic lice, scabies (parasites).

Transmission routes:

Vaginal sex (most common).
Anal or oral sex.
BLOOD-to-blood (sharing needles, blood transfusions before screening).
MOTHER to BABY (during birth or breast milk).

NOT transmitted by:

Hugging, kissing (most STIs).
Sharing food, drink, cutlery.
Toilet seats.
Mosquitoes.

Worked qualitative. Why are STIs called 'silent' diseases?

Many cause no obvious symptoms early on.
Person may not know they're infected → continues to transmit.
Routine testing important for sexually active people.

Cambridge tip. Cambridge mainly tests HIV. Know the basic facts about it.

STI: passed by sexual contact.
Bacterial (curable) vs viral (manageable).
Routes: sex, blood, mother-to-baby.
Not by casual contact.

HIV and AIDS

HIV = the virus. AIDS = the late stage when immune system fails. Manageable but not curable.

HIV = Human Immunodeficiency Virus. A retrovirus.

What HIV does inside the body:

Enters blood via sex, shared needles, or mother-to-baby.
Targets HELPER T-LYMPHOCYTES (a type of WBC central to immune response).
Inserts itself into the T-cell DNA → uses cell to make more virus.
Destroys T-cells in the process.
Over years, T-cell count drops → immune system progressively WEAKENS.

The progression to AIDS:

Stage	What's happening
HIV infection	Virus present; few or no symptoms
Asymptomatic phase	Years, even decades; T-cells slowly declining
Symptomatic HIV	Frequent minor infections; weight loss; tiredness
AIDS	T-cell count very low; opportunistic infections + cancers; without treatment, death within 1-3 years

Opportunistic infections.

Diseases that healthy immune systems easily handle.
Become deadly in AIDS patients: pneumonia, TB, certain cancers (Kaposi's sarcoma), severe fungal infections.
They take advantage of the weakened defences.

Worked qualitative. Why is the HIV pandemic harder to fight than something like measles?

HIV mutates rapidly → hard to make a vaccine.
It hides INSIDE host DNA → hard to eliminate.
Long latent phase → people transmit before knowing.
Stigma + cultural barriers slow testing and treatment.
Despite decades of research, no vaccine yet (though great treatments).

Cambridge tip. Memorise: HIV destroys T-LYMPHOCYTES (or 'helper T-cells'). Saying 'attacks immune system' is too vague.

HIV: virus.
Destroys helper T-lymphocytes.
Years of slow decline → AIDS.
Opportunistic infections kill.
Manageable with drugs; no cure yet.

Preventing HIV transmission

Condoms, clean needles, blood screening, treatment-as-prevention. Education matters.

1. Condoms.

Latex or polyurethane barrier.
Stops body fluid contact during sex.
Protect against HIV AND most other STIs (and unwanted pregnancy).
Most effective single prevention method.

2. Don't share needles.

Major transmission route in injecting drug users.
Needle exchange programs (give clean needles in exchange for used) reduce spread.
Tattoos and piercings: only use sterilised equipment.

3. Blood screening.

All blood donations now tested for HIV before use.
Eliminated transmission via transfusion in countries with screening.

4. Antiretroviral therapy (ART).

Drugs that block different stages of HIV's life cycle.
Reduce viral load to UNDETECTABLE → infected person can't transmit.
Lifelong treatment; not a cure but life expectancy near normal.

5. PrEP (pre-exposure prophylaxis).

Antiretroviral drugs for HIV-NEGATIVE people at high risk.
Reduces chance of getting HIV by ~99% if taken daily.

6. Mother-to-baby prevention.

HIV+ pregnant women given antiretrovirals.
Reduces transmission to baby from ~25% to <1%.
Baby may also be given prophylactic drugs.

7. Education and testing.

Knowing your status: treatment can start early.
Reducing stigma → more people test.
Comprehensive sex education in schools.

Worked qualitative. Why is breastfeeding controversial for HIV+ mothers?

HIV can transmit through breast milk.
BUT formula feeding requires clean water + steady supply (often unavailable in poor regions).
WHO recommendation: HIV+ mothers in low-resource settings on antiretrovirals should still breastfeed (ART reduces transmission risk dramatically).
High-resource settings: formula feeding usually safer.
A trade-off between two risks.

Cambridge tip. When asked HOW to limit HIV, give 4-5 strategies. Cambridge marks each.

Condoms (most effective).
Clean needles.
Blood screening.
Antiretroviral drugs (treatment + PrEP).
Education + testing.

How it’s examined

STIs appear Paper 4 (3-5 marks: HIV mechanism + prevention). Examiner reports flag students confusing HIV with AIDS, claiming HIV is curable, and missing 'T-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.

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Step-by-step worked examples — Sexually Transmitted Infections

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.

1What are STIs?
Core• definition
Question
Define an STI and give THREE examples.
Step-by-step solution
1. Step 1
  STI (sexually transmitted infection) = disease passed from person to person by SEXUAL contact (vaginal, oral, anal sex; sometimes blood, breast milk).
2. Step 2
  Examples:
3. Step 3
  HIV (virus, leads to AIDS).
4. Step 4
  Gonorrhoea (bacterium).
5. Step 5
  Syphilis (bacterium); chlamydia (bacterium); hepatitis B (virus); herpes (virus).
Answer
STI = disease via sexual contact. Examples: HIV, gonorrhoea, syphilis, chlamydia, herpes.
2HIV — what it does
Extended• Adapted from 0610/42 Oct/Nov 2024 Q16• HIV
Question
Explain how HIV affects the body and leads to AIDS.
Step-by-step solution
1. Step 1
  HIV = human immunodeficiency virus.
2. Step 2
  Targets and DESTROYS specific WHITE BLOOD CELLS (helper T-lymphocytes) — central to immune system.
3. Step 3
  Over years, T-cell count drops → immune system weakens.
4. Step 4
  Eventually too weak to fight off infections that healthy people easily resist (opportunistic infections like pneumonia, TB, certain cancers).
5. Step 5
  This advanced stage is AIDS (Acquired Immunodeficiency Syndrome).
Answer
HIV destroys helper T-lymphocytes → immune system fails → opportunistic infections + cancers → AIDS.
3How to prevent HIV transmission
Extended• HIV prevention
Question
Describe FOUR ways to LIMIT HIV transmission.
Step-by-step solution
1. Step 1
  Use CONDOMS during sex — physical barrier between body fluids.
2. Step 2
  Don't share NEEDLES (drug use) — direct blood-to-blood contact.
3. Step 3
  Screen blood donations — only HIV-free blood used in transfusions.
4. Step 4
  Antiretroviral drugs can suppress viral levels in infected people; less risk of transmitting; PrEP (pre-exposure prophylaxis) for high-risk uninfected people.
5. Step 5
  Education + testing to know status; treatment of pregnant infected women prevents mother-to-baby transmission.
Answer
Condoms, no needle sharing, blood screening, antiretroviral drugs (treatment + PrEP), education/testing.

Key Definitions and Keywords — Sexually Transmitted Infections

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.

STI (sexually transmitted infection)
Examiner keyword
Disease caused by a pathogen passed from person to person by SEXUAL contact (or sometimes blood / mother-to-baby).
HIV
Examiner keyword
Human immunodeficiency virus. Destroys white blood cells (T-lymphocytes), causing immune deficiency.
AIDS
Examiner keyword
Acquired Immunodeficiency Syndrome. Late-stage HIV infection with severely weakened immune system; opportunistic infections.

Common Mistakes and Misconceptions — Sexually Transmitted 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.

✕Using HIV and AIDS interchangeably.
Why it happens
Both related.
How to avoid it
HIV is the VIRUS. AIDS is the SYNDROME caused by the virus when it's destroyed enough immune cells. People can be HIV+ for years without having AIDS, especially with treatment.
✕Thinking HIV spreads through casual contact (sharing food, hugging).
Why it happens
Stigma + misinformation.
How to avoid it
HIV needs DIRECT BLOOD/SEMEN/VAGINAL FLUID/BREAST MILK contact. Not via shared cups, hugs, kisses, mosquitoes, or sneezes.
✕Saying HIV is curable.
Why it happens
Modern treatments are very effective.
How to avoid it
HIV is currently INCURABLE — drugs SUPPRESS the virus to undetectable levels (manageable lifelong condition) but don't eliminate it. Major research goal.

Practice questions

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Sexually Transmitted Infections — frequently asked questions

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Sexually Transmitted Infections

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1What are STIs?

2HIV — what it does

3How to prevent HIV transmission

STI (sexually transmitted infection)

HIV

AIDS

✕Using HIV and AIDS interchangeably.

✕Thinking HIV spreads through casual contact (sharing food, hugging).

✕Saying HIV is curable.

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Sexually Transmitted Infections — frequently asked questions