Biology · 16 Reproduction

16.6 Sexually transmitted infections

Before you start

A pathogen is a disease-causing organism, and a transmissible disease is one in which the pathogen is passed from one host to another.
Viruses can only reproduce inside a host cell, which is why they cannot be treated with antibiotics.
White blood cells, including lymphocytes, defend the body against pathogens.
The placenta connects the developing fetus to the mother, and substances pass across it.
Percentage change = (change ÷ original) × 100.

Syllabus objectives covered (5)

Describe a sexually transmitted infection (STI) as an infection that is transmitted through sexual contact
State that human immunodeficiency virus (HIV) is a pathogen that causes an STI
State that HIV infection may lead to AIDS
Describe the methods of transmission of HIV
Explain how the spread of STIs is controlled

Part 1 of 5

What 'sexually transmitted' actually classifies

An STI is an infection that is transmitted through sexual contact. That sentence is the objective, and it is also the answer — a one-mark Define here is testing whether you can produce it cleanly.
Notice what the definition is based on. It is not the type of pathogen and it is not which organs are affected. It is the route — how the pathogen gets from one person to another.
Compare with routes you already know. Influenza is transmitted in droplets through the air. Cholera is transmitted in contaminated water. Those are classified by route too, and 'sexually transmitted' sits in exactly the same list.
For 0610 the one STI you must know by name is HIV. Other STIs exist and several are caused by bacteria; you are not required to name them, but it is worth knowing they exist, because it explains why antibiotics appear in the control measures later — they work against bacterial STIs and are useless against HIV.

Part 2 of 5

HIV is a pathogen, and it attacks the immune system itself

HIV stands for human immunodeficiency virus. It is a pathogen — an organism that causes disease — and the disease it causes is transmitted by sexual contact, so it causes an STI.
Be exact about what kind of pathogen: HIV is a virus. That single fact does more work in this topic than anything else, because it explains why antibiotics cannot cure it.
HIV infects lymphocytes — white blood cells that are central to the immune response. The virus reproduces inside them and destroys them, so over time the number of working lymphocytes in the blood falls.
Follow the consequence through. Fewer lymphocytes means antibodies are not produced in the numbers needed, so the immune system can no longer defend the body against other pathogens.
This takes years, and here is the part that matters most for control: an infected person may have no symptoms at all for a long time, feel completely well, and still pass the virus on. That is why control depends on testing rather than on waiting for someone to look ill.

Part 3 of 5

HIV and AIDS are two different words

HIV is the virus. AIDS — acquired immune deficiency syndrome — is the condition that may develop later, when so many lymphocytes have been destroyed that the immune system can no longer protect the person.
A person with AIDS becomes ill with infections and some cancers that a healthy immune system would normally fight off. They are not dying of HIV directly; they are dying of what HIV has left them defenceless against.
The syllabus wording is 'HIV infection may lead to AIDS', and that word 'may' is deliberate. With antiretroviral drugs, an infected person can stay healthy for many years and may never develop AIDS.
Two phrasings to strike out of your writing: 'the AIDS virus' and 'she caught AIDS'. You catch HIV. AIDS is a condition that HIV may cause. If a question asks what causes AIDS, the answer is HIV.

Part 4 of 5

How HIV gets from one person into another

One rule sits underneath every route: HIV is carried in body fluids, and it must get into the other person's body — into their blood. There is no airborne route, and it cannot cross unbroken skin.
The body fluids to name are blood, semen, vaginal fluids and breast milk. Naming a fluid is often worth a marking point on its own.
Route 1 — sexual contact. During unprotected sexual intercourse, semen or vaginal fluid containing the virus comes into contact with the lining of the partner's reproductive tract, and the virus enters their blood.
Route 2 — blood to blood. Sharing needles or syringes, most commonly among people who inject drugs; transfusion of infected blood or blood products; and re-used or unsterilised surgical, dental or tattooing instruments.
Route 3 — mother to child. Across the placenta during pregnancy, during birth as the baby passes through the birth canal, and after birth through breast milk.
Now use the rule in reverse, because examiners ask this as a Suggest. HIV is not transmitted by hugging, shaking hands, sharing cups or cutlery, toilet seats, coughs and sneezes, or insect bites — none of those put infected body fluid into another person's blood.

Part 5 of 5

Controlling the spread — pair each measure with the route it blocks

Every control measure exists to block one of those routes. If you write your answer as 'measure — the route it blocks', you turn a list into an explanation, and the command word here is Explain.
Blocking the sexual route. Using a condom during sexual intercourse puts a barrier between the partners so that semen and vaginal fluid do not come into contact. Limiting the number of sexual partners reduces the chance that any partner is infected. Not having sexual contact prevents transmission by that route entirely.
Blocking the blood route. Not sharing needles, and supplying sterile needles through needle-exchange schemes. Screening all donated blood for HIV before it is transfused. Sterilising surgical and dental instruments, and using disposable ones.
Blocking mother-to-child transmission. Testing during pregnancy, and treating infected mothers with antiretroviral drugs, which greatly reduces the chance the baby is infected. Where safe, clean formula feed is available, bottle feeding avoids transmission through breast milk.
Finding the infections nobody can see. Since an infected person may have no symptoms, control depends on testing and screening programmes, and on contact tracing — finding and testing the sexual contacts of an infected person so they can be treated and stop passing it on.
Treating people who are infected. Bacterial STIs can be cured with antibiotics, and a cured person can no longer transmit them. HIV cannot be cured this way, because antibiotics act on bacteria and have no effect on viruses. Antiretroviral drugs instead reduce the amount of virus in the blood, which keeps the person healthy and makes them much less likely to transmit it.
Education. None of the above happens unless people know how the infection is transmitted and what protects them, so health education programmes are a legitimate answer — but only if you say what the education is about.
What does not score: 'be careful', 'practise safe sex', 'be sensible'. Those are not measures, they are attitudes. Name the measure and name what it stops.

Worked examples

Example 1. A health clinic finds that most new HIV infections in one town are among people who inject drugs, and among the babies of infected mothers. Suggest three measures the clinic could take, and explain how each would reduce transmission.

Show the working

Start by identifying the routes involved, because the measures have to match the routes. Injecting drugs points to blood-to-blood transmission through shared needles. Infected mothers point to transmission across the placenta, during birth, or in breast milk.
Take the needle route first. The transmission happens because blood from one person enters another's bloodstream on a used needle, so supplying free sterile needles removes the reason to share one.
Now the mother-to-child route. The clinic cannot change the fact of pregnancy, so it must reduce how much virus is present and remove one route: test pregnant women, treat those who are infected with antiretroviral drugs to lower the amount of virus in the blood.
Take the third route separately — breast milk. Advising bottle feeding where clean water and formula are reliably available removes that route after birth.
A fourth measure worth adding, if the question allows it, is contact tracing and testing, because infected people with no symptoms cannot be helped until they know.

Answer: Provide sterile needles through a needle-exchange scheme, so that blood from an infected person is not injected into another person. Test pregnant women and treat those who are HIV positive with antiretroviral drugs, reducing the amount of virus that can cross the placenta or reach the baby during birth. Advise bottle feeding where safe formula is available, removing transmission through breast milk.

Example 2. A man was infected with HIV eight years ago. He had no symptoms for six years. He now has a very low number of lymphocytes in his blood and is suffering from repeated chest infections that do not clear up. Explain what has happened, using the terms HIV and AIDS correctly.

Show the working

Begin with what HIV does at the cell level, because everything else follows from it. HIV infects and destroys lymphocytes, reproducing inside them.
Explain the six symptom-free years rather than ignoring them. The number of lymphocytes falls slowly, so for a long time enough remain for the immune system to work normally — which is why he felt well and why he could have transmitted the virus without knowing.
Now connect the low lymphocyte count to the chest infections. With too few lymphocytes, antibodies are not produced in the amounts needed, so the immune system cannot defend him against other pathogens.
Name the stage correctly. Repeated infections that a healthy immune system would clear are what defines the condition AIDS, which HIV infection may lead to.
Check your wording before you finish. The pathogen he caught is HIV; AIDS is the condition he has now developed. Writing 'he caught AIDS eight years ago' would contradict your own explanation.

Answer: HIV, a virus, has infected and destroyed his lymphocytes over eight years. While enough lymphocytes remained he had no symptoms, although he could still transmit the virus. Now that the lymphocyte count is very low his immune system cannot defend him against other pathogens, so infections persist — HIV infection has led to AIDS.

Example 3. A region recorded 4800 new HIV infections in one year. Five years after starting a programme of free condoms, free testing and contact tracing, it recorded 3120 new infections in a year. Calculate the percentage decrease in new infections, and suggest how each part of the programme contributed.

Show the working

Find the change first, because percentage change is always change compared with the original: $4800 - 3120 = 1680$ fewer infections.
Divide by the original value, not the new one. The original is 4800, the figure before the programme.
$percentage change = \frac{1680}{4800} \times 100$
Work it out: $1680 \div 4800 = 0.35$ , and $0.35 \times 100 = 35$ . The unit is the per cent sign, and leaving it off costs the final mark.
Now the biology. Free condoms block the sexual route directly, by keeping semen and vaginal fluid from contacting the partner.
Free testing works on the invisible part of the problem: people with no symptoms find out they are infected, so they can be treated and can avoid transmitting it.
Contact tracing extends testing to the people most likely to be infected already — the sexual contacts of someone who has tested positive — so they are found before they transmit it further.

Answer: A 35% decrease. Condoms act as a barrier preventing contact between body fluids during intercourse; testing identifies infected people who have no symptoms so they can be treated and take precautions; contact tracing finds and tests their sexual contacts before those people transmit the virus further.

Where people go wrong

Mistake: Writing 'the AIDS virus' or 'he caught AIDS'.

Instead: HIV is the virus that is caught. AIDS is the condition that HIV infection may lead to.

The two objectives are separate for a reason, and the distinction is exactly what a two-mark question is testing. Using the words interchangeably shows you cannot make the distinction, so the mark goes.

Mistake: Thinking HIV can be caught from sharing a cup, a toilet seat or a mosquito bite.

Instead: Transmission needs infected body fluid — blood, semen, vaginal fluid or breast milk — to enter another person's body.

Everyday contact does not do that. Working from the rule rather than from a memorised list lets you answer a Suggest about any situation the examiner invents.

Mistake: Suggesting antibiotics to treat or control HIV.

Instead: HIV is a virus, and antibiotics act on bacteria. Antiretroviral drugs are used instead; they reduce the amount of virus but do not cure the infection.

This is a favourite one-mark check. Antibiotics are a valid control measure for bacterial STIs, so the mark depends on you knowing which kind of pathogen you are dealing with.

Mistake: Answering 'how is the spread controlled?' with 'be careful' or 'practise safe sex'.

Instead: Name a specific measure and the route it blocks: 'use a condom, so that semen and vaginal fluid do not come into contact'.

The command word is Explain. A measure with no mechanism is at best half a marking point, and vague moral advice is not a marking point at all.

Mistake: Assuming a person with HIV always looks ill, so an infected partner would be obvious.

Instead: A person can be infected, have no symptoms for years, feel well, and still transmit the virus.

This is why testing, screening and contact tracing are control measures rather than optional extras — and questions about why an STI spreads quickly usually want exactly this point.

Check yourself

Define a sexually transmitted infection.
Answer
An infection that is transmitted through sexual contact.
What kind of pathogen is HIV, and which cells does it infect?
Answer
HIV is a virus. It infects lymphocytes, the white blood cells involved in the immune response, and destroys them.
State the relationship between HIV and AIDS in one sentence.
Answer
HIV is the virus that infects a person, and HIV infection may lead to AIDS, the condition in which the damaged immune system can no longer defend the body against other pathogens.
Give three ways in which HIV can be transmitted from an infected mother to her baby.
Answer
Across the placenta during pregnancy, during birth, and through breast milk.
Explain why HIV cannot be transmitted by sharing a drinking glass.
Answer
Transmission requires infected body fluid such as blood, semen, vaginal fluid or breast milk to enter another person's body. Sharing a glass does not do this.
Name two control measures that block the blood-to-blood route, and say what each one stops.
Answer
Screening donated blood before transfusion, so infected blood is not given to a patient; and not sharing needles, or supplying sterile needles, so blood from an infected person is not injected into someone else.
Why is contact tracing useful in controlling an STI?
Answer
Because an infected person may have no symptoms. Tracing and testing the sexual contacts of someone who tests positive finds infected people who do not know, so they can be treated and stop transmitting it.
Why are antibiotics part of the control of some STIs but not of HIV?
Answer
Antibiotics kill bacteria, so they cure bacterial STIs and a cured person can no longer transmit them. HIV is a virus, so antibiotics have no effect on it; antiretroviral drugs are used instead.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1.

(a) Define the term sexually transmitted infection. 1 mark No calculator

Mark scheme

B1 an infection that is transmitted / spread by sexual contact

(b) State the name of the pathogen that causes AIDS and the type of pathogen it is. 2 marks No calculator

Mark scheme

B1 human immunodeficiency virus / HIV
B1 a virus

(c) Describe the difference between HIV and AIDS. 2 marks No calculator

Mark scheme

B1 HIV is the virus / pathogen, which infects and destroys lymphocytes
B1 AIDS is the condition that HIV infection may lead to, in which the immune system can no longer defend the body against other pathogens. CON if the answer also refers to 'the AIDS virus'

Question 2. A health clinic records the ways in which people attending it became infected with HIV. Some of them had no symptoms and were only found to be infected when they were tested.

(a) Describe three ways in which HIV may be transmitted from one person to another. 3 marks No calculator

Mark scheme

B1 unprotected sexual intercourse / contact with semen or vaginal fluid of an infected person
B1 blood to blood — sharing needles or syringes / transfusion of infected blood / unsterilised surgical instruments
B1 from mother to child — across the placenta / during birth / in breast milk

(b) Suggest why a person who feels completely well may still transmit HIV to others. 2 marks No calculator

Mark scheme

B1 symptoms may not appear for years after infection
B1 the virus is still present in the body fluids and can be passed on, and the person does not know to take precautions or seek treatment

(c) Explain two measures that would reduce transmission of HIV through blood. 2 marks No calculator

Mark scheme

B1 screen / test donated blood before transfusion, so infected blood is not given to a patient
B1 do not share needles / provide sterile needles or a needle exchange, so blood from an infected person is not injected into another person. Accept sterilising or using disposable surgical instruments

Question 3. A region recorded 4800 new HIV infections in 2018. A programme of free condoms, free testing and contact tracing was introduced. In 2023 the region recorded 3120 new infections.

(a) Calculate the percentage decrease in the number of new infections between 2018 and 2023. 2 marks

Mark scheme

C1 (4800 - 3120) / 4800 × 100 or 1680 / 4800 × 100 seen
A1 35%

(b) Explain how the use of condoms reduces the transmission of STIs. 2 marks No calculator

Mark scheme

B1 acts as a barrier during sexual intercourse
B1 so semen and vaginal fluid, which may contain the pathogen, do not come into contact with the partner

(c) Some STIs are treated with antibiotics, but HIV is not. Explain why. 2 marks No calculator

Mark scheme

B1 HIV is a virus
B1 antibiotics act on / kill bacteria and have no effect on viruses

How this is examined

Naming control measures and saying which route each one blocks: a four- to six-mark Explain — 'Explain how the spread of STIs can be controlled' — sometimes with a short passage or a table of infection figures in front of it One marking point per measure, each with its mechanism: 'use of condoms, which prevent contact between body fluids during intercourse'; 'limiting the number of sexual partners, reducing the chance a partner is infected'; 'screening / testing donated blood before transfusion'; 'not sharing needles / needle exchange'; 'testing and contact tracing, because infected people may have no symptoms'; 'treating bacterial STIs with antibiotics so they are no longer transmitted'; 'health education about how the infection is spread'. Bare measures with no mechanism cap you at about half the marks.
The three transmission routes for HIV: a three-mark Describe, or a Suggest asking whether a described situation could transmit HIV 'unprotected sexual intercourse / contact with semen or vaginal fluid'; 'blood to blood — sharing needles, transfusion of infected blood, unsterilised instruments'; 'mother to child — across the placenta, during birth, in breast milk'. For the Suggest version, the reason scores: infected body fluid must enter the other person's body, which casual contact does not achieve.
The difference between HIV and AIDS: a two-mark Describe or State, often as the opening part of a longer question 'HIV is a virus / pathogen that infects and destroys lymphocytes' and 'AIDS is the condition that may develop when the immune system is so damaged that it can no longer defend the body against other pathogens'. Writing 'the AIDS virus' anywhere in the answer contradicts the distinction and loses it.
Why an infected person with no symptoms matters for control: a two-mark Suggest, typically 'Suggest why the number of infections continued to rise even though a treatment was available' 'A person may be infected for years with no symptoms' plus 'so they do not know they are infected and can transmit the virus / do not seek testing or treatment'. This is also the reason to give if asked why screening and contact tracing are needed.
What makes an infection 'sexually transmitted': a one-mark Define or State, sometimes followed by 'Name one STI' 'An infection transmitted through sexual contact' — the classification is by route, not by pathogen. HIV is the named example the syllabus expects.

Words to know

Sexually transmitted infection (STI): An infection that is transmitted through sexual contact.
HIV: Human immunodeficiency virus — the virus that infects and destroys lymphocytes, and which causes an STI.
AIDS: Acquired immune deficiency syndrome — the condition that HIV infection may lead to, in which the damaged immune system can no longer defend the body against other pathogens.
Lymphocyte: A type of white blood cell involved in the immune response, including the production of antibodies. These are the cells HIV destroys.
Contact tracing: Finding and testing the sexual contacts of a person who has tested positive, so that infected people who have no symptoms can be treated.
Antiretroviral drug: A drug that reduces the amount of HIV in the blood. It does not cure the infection, but it keeps the person healthy for longer and makes transmission less likely.
Screening: Testing people, or donated blood, for an infection before symptoms appear or before the blood is used.

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