Sexually Transmitted Infections

This lesson covers sexually transmitted infections (STIs) as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand what STIs are, how they are spread, and the specific examples of HIV and chlamydia, including methods of prevention and reducing spread.

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What Are Sexually Transmitted Infections?

Sexually transmitted infections (STIs) are infections that are spread through sexual contact (vaginal, oral or anal sex). They are caused by pathogens — bacteria, viruses or other microorganisms — that are passed from one person to another during intimate physical contact.

STIs are a significant public health concern worldwide. Many STIs can be prevented through simple measures, and early treatment can prevent serious complications.

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HIV (Human Immunodeficiency Virus)

HIV was covered in detail in the previous lesson on viruses, but here we focus on it as an STI.

How HIV Is Transmitted

Route of transmission	Detail
Sexual contact	Unprotected vaginal, anal or oral sex with an infected person
Sharing needles	Contaminated needles used for injecting drugs
Mother to child	During pregnancy, childbirth or breastfeeding
Blood products	Receiving contaminated blood (now rare due to screening)

How HIV Causes Disease

1. HIV infects T helper lymphocytes (CD4 cells) — a type of white blood cell that is essential for coordinating the immune response.
2. The virus uses the host cell's machinery to replicate, then destroys the cell.
3. Over time, the number of T helper cells decreases.
4. When T helper cell numbers drop below a critical level, the immune system can no longer fight infections effectively.
5. This late stage is called AIDS (Acquired Immune Deficiency Syndrome).
6. People with AIDS are vulnerable to opportunistic infections — infections that a healthy immune system would normally fight off easily (e.g. pneumonia, tuberculosis, certain cancers).

graph TD
    A[HIV enters body] --> B["HIV infects<br/>T helper lymphocytes"]
    B --> C["Virus replicates<br/>inside T cells"]
    C --> D["T helper cells<br/>are destroyed"]
    D --> E["Immune system<br/>becomes weaker"]
    E --> F{"T cell count<br/>critically low?"}
    F -->|Yes| G["AIDS develops<br/>Opportunistic infections"]
    F -->|No| H["Antiretroviral drugs<br/>control virus levels"]

Controlling HIV

Method	How it works
Antiretroviral drugs	A combination of drugs that suppress HIV replication, keeping viral levels low and T cell counts high; person can live a normal lifespan
Condoms	Barrier method that prevents exchange of body fluids during sex
Needle exchange programmes	Provide clean needles to prevent sharing of contaminated equipment
Blood screening	All donated blood is tested for HIV before use
PrEP (pre-exposure prophylaxis)	Antiretroviral drugs taken by HIV-negative people at high risk to prevent infection

Exam Tip: Remember that antiretroviral drugs do NOT cure HIV — they control the virus. The person remains infected for life but can live healthily and reduce the risk of transmitting the virus.

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Chlamydia

Chlamydia is the most common bacterial STI in the UK, particularly among young people aged 15–24.

Feature	Detail
Type of pathogen	Bacterium (Chlamydia trachomatis)
How it spreads	Unprotected sexual contact (vaginal, anal, oral); can also be passed from mother to baby during birth
Symptoms	Often asymptomatic (no symptoms), especially in early stages. When symptoms occur: abnormal discharge, pain when urinating, pelvic pain
Complications if untreated	In women: pelvic inflammatory disease (PID), which can lead to infertility. In men: infection of the testes, which can also cause infertility
Treatment	Easily treated with antibiotics (usually a course of azithromycin or doxycycline)
Screening	The UK National Chlamydia Screening Programme offers free, confidential testing for people aged 15–24

Why Chlamydia Is a Major Concern

• Many infected people have no symptoms, so they do not know they are infected and can unknowingly spread it to sexual partners.
• If left untreated, it can cause serious long-term damage, including infertility.
• It is easily treated with antibiotics if detected early — this is why screening programmes are so important.

Exam Tip: The fact that chlamydia is often asymptomatic is a key exam point. If asked why chlamydia spreads so easily, the answer is that many people do not know they are infected because they have no symptoms.

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Preventing STIs

Prevention strategies for STIs can be grouped into several categories:

Strategy	Detail
Barrier methods	Using condoms during sex prevents the exchange of body fluids and provides protection against most STIs
Regular screening	Getting tested regularly, especially when changing sexual partners, allows early detection and treatment
Treatment of infected individuals	Prompt treatment (e.g. antibiotics for chlamydia) reduces the chance of spreading the infection
Contact tracing	Notifying sexual partners of an infected person so they can be tested and treated
Education	Teaching people about the risks of STIs and how to prevent them
Vaccination	Some STIs can be prevented by vaccination — e.g. the HPV vaccine protects against human papillomavirus, which can cause cervical cancer

Comparing HIV and Chlamydia

Feature	HIV	Chlamydia
Pathogen type	Virus	Bacterium
Curable?	No (controlled with antiretrovirals)	Yes (antibiotics)
Symptoms	Flu-like initially; later AIDS	Often asymptomatic
Long-term effects	AIDS, opportunistic infections	Infertility if untreated
Prevention	Condoms, PrEP, clean needles	Condoms, screening
UK prevalence	Relatively low (but significant globally)	Most common bacterial STI in UK

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Summary