Sexually transmitted infectionsSpec 5.8
In short
Sexually transmitted infections (STIs) are spread by sexual contact. Chlamydia is caused by a bacterium and can cause infertility, while HIV is a virus that destroys white blood cells and can lead to AIDS. Using condoms, which act as a barrier to body fluids, screening and testing, and not sharing needles all reduce the spread.
Sexually transmitted infections (STIs) are spread by sexual contact. You need two examples: chlamydia and HIV.
| Chlamydia | HIV | |
|---|---|---|
| Pathogen | Bacterium | Virus |
| How it spreads | Unprotected sexual contact | Unprotected sexual contact; sharing needles; contact with infected blood; from an infected mother to her baby |
| Effects | Often no symptoms; can cause infertility | Destroys white blood cells and can lead to AIDS |
| How spread can be reduced | Use condoms; screening and testing; treating infected people with antibiotics; informing partners | Use condoms; do not share needles; screen donated blood; drug treatment to lower the amount of virus |
Condoms act as a barrier that stops body fluids passing from one person to another. Using a condom, having fewer partners and being tested regularly all reduce the spread.
A good answer to 'how can the spread of an STI be reduced' names the method and says how it works, for example 'use a condom, which prevents body fluids passing between partners'.
Quick check
Name the four types of pathogen.
Show answer
Viruses, bacteria, fungi and protists.
How is cholera spread, and how can its spread be reduced?
Show answer
Through contaminated water; provide clean water and treat sewage.
What is the vector for malaria?
Show answer
The mosquito.
Triple only: what happens to the host cell in the lytic pathway?
Show answer
It bursts open (lyses), releasing new viruses.
Name a bacterial STI and a viral STI.
Show answer
Chlamydia (bacterial) and HIV (viral).
Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026