Decision-Making Regarding Sexual Relationships
Senior 1 · Biology
Decision-Making Regarding Sexual Relationships
Lesson 3 of 5
25 min
Two young adults in a health-centre waiting room both look fit and well. One has an infection that could harm her fertility; the other has been living with HIV for eight years and is completely healthy because he takes his medicine every day. Neither shows any sign.
This lesson gives you the facts about sexually transmitted infections (STIs), including HIV, as health authorities such as the WHO state them: how they spread, how they do not spread, and how the chain of transmission can be broken.
Definition
Sexually transmitted infection (STI): an infection passed from one person to another mainly through sexual contact. Some STIs can also pass from mother to baby, or through blood.
HIV (human immunodeficiency virus): a virus that attacks white blood cells of the immune system. AIDS (acquired immunodeficiency syndrome): the most advanced stage of untreated HIV infection, when the immune system is so weak that serious infections and some cancers develop.
Asymptomatic: having an infection but showing no signs (symptoms) of it.
The WHO counts more than 30 different bacteria, viruses and parasites that can be passed on through sexual contact. Most STIs cause no signs at all in many people, so they can be passed on without anyone knowing. Only a test can show whether a person is infected.
| STI (cause) | Possible signs (often none) | Treatment |
|---|---|---|
| HIV (virus) | often none for years; later weight loss, fever, repeated infections | no cure; lifelong antiretroviral therapy (ART) controls it |
| Syphilis (bacterium) | a painless sore; later a rash | curable with antibiotics |
| Gonorrhoea (bacterium) | unusual discharge; pain when passing urine | curable, but drug resistance is rising |
| Chlamydia (bacterium) | usually none; can cause infertility if untreated | curable with antibiotics |
| Trichomoniasis (parasite) | unusual discharge, itching | curable |
| Genital herpes (virus) | painful blisters or sores that come and go | no cure; medicines control it |
| HPV and hepatitis B (viruses) | usually none; some types of HPV cause cervical cancer; hepatitis B damages the liver | no cure for the viruses; vaccines prevent them |
Prevention for all of them: abstinence; for adults, condoms used correctly every time and faithfulness to one tested partner; and testing and treatment of both partners.
STIs matter even when they cause no signs: untreated, some cause infertility, harm babies during pregnancy and birth, or cause cancer, and several of them make HIV infection more likely.
| HIV CAN pass through | HIV does NOT pass through |
|---|---|
| unprotected vaginal or anal sex with a person living with HIV | hugging, kissing, shaking hands |
| sharing needles, syringes, razor blades or other sharp tools with infected blood on them | sharing food, water, cups or plates |
| from a mother to her baby during pregnancy, birth or breastfeeding | toilets, swimming, sitting together in class |
| unsafe blood transfusions, injections or cutting and piercing with unsterile tools | mosquito bites, coughs or sneezes |
The WHO estimated that about 41.0 million people were living with HIV at the end of 2025, 64 percent of them in the WHO African Region. There is still no cure, but HIV is now a manageable long-term condition: people who take antiretroviral therapy (ART) every day can live long, healthy lives, go to school, work and raise families. People living with HIV who take ART and have an undetectable amount of virus in their blood do not pass HIV to their sexual partners, and ART given to pregnant women protects their babies.
Definition
Antiretroviral therapy (ART): daily medicines that stop HIV from multiplying, so the immune system can recover. Undetectable: having so little virus in the blood that tests cannot find it.
PEP (post-exposure prophylaxis): HIV medicines taken for 28 days after a possible exposure, started as soon as possible, ideally within 24 hours and no later than 72 hours. PrEP (pre-exposure prophylaxis): HIV medicines taken before possible exposure by HIV-negative adults at high risk.
Window period: the time after infection before a test can detect it. Most people make antibodies to HIV within 28 days, so a test during this time may be negative even though the person is infected.
For HIV to pass from one person to another, a chain of events must happen. Each link is a place where the chain can be broken.
Figure 1: The chain of HIV transmission and how to break it. Facts: WHO, 2024 and 2026.
Worked example 1: which link is broken?
| Action | Link broken |
|---|---|
| A pregnant woman living with HIV takes ART and her baby is born HIV-negative. | 3 (way across: mother to baby), and 1 (the source's virus is controlled) |
| A pupil helping an injured friend covers her hands with a plastic bag before touching the bleeding wound. | 2 (no contact with blood) |
| A young person chooses to abstain from sex. | 3 (no sexual route) |
| A nurse pricked by a used needle starts PEP within a few hours. | 4 (stops the virus establishing itself in the new host) |
Worked example 2: the window period
An adult had a possible exposure to HIV on 3 March and tested negative on 13 March. Is the result reliable? When should they test again?
Answer: not yet reliable; test again on or after 31 March. Meanwhile, avoid anything that could pass HIV on. (A health worker would also have offered PEP if they had come within 72 hours of the exposure.)
Worked example 3: evaluating claims
Decide whether each claim is true, and give the evidence.
How close are we to eradicating HIV? (TED-Ed, 5 min). Watch for how ARVs work, for PrEP, and for the idea that 'undetectable means untransmittable'. The video is from 2019, so its figures are older than those in this lesson.
Stigma means negative beliefs and unfair treatment of people because of something about them, such as living with HIV. Stigma makes people afraid to get tested or to take their medicine, which harms everyone. Because HIV is not spread by everyday contact, there is no reason to avoid, exclude or gossip about anyone. A person's HIV status is private; it is theirs to share, not yours.
Warning
Common mistakes. (1) "Healthy-looking people cannot have STIs": most infections have no signs. (2) "HIV and AIDS are the same thing": AIDS is the advanced stage of untreated HIV. (3) "A negative test the day after a risk means I am safe": for most people the window period is up to 28 days. (4) "HIV is a death sentence": with ART, people live long, healthy lives.
Tip
Memory hook: "Four fluids, four links." HIV travels in blood, semen, vaginal fluids and breast milk; break the chain at the source, the fluid, the way across, or the new host.
Practice: STIs and HIV
Key points
Sources and credits
Written by Ganzaa: the lesson text, Figure 1, the tables, the worked examples and the practice questions.
