5.8 - Sexually transmitted infections
Explain how Chlamydia and HIV can pass during sexual contact. Distinguish infection from symptoms, compare barrier prevention with testing and treatment, and explain why preventing pregnancy does not necessarily prevent infection.
5.8 · Sexually transmitted infections
A sexually transmitted infection (STI) is an infection that can pass between people through sexual contact. The route may involve infected semen, vaginal fluid or blood reaching another person's mucous membranes or broken skin. A contraceptive method prevents an STI only if it interrupts that route: the contraceptive pill can prevent pregnancy, but it does not form a barrier to pathogen transfer.
| STI | Pathogen and effect | Sexual spread | Reducing or preventing spread |
|---|---|---|---|
| Chlamydia | A bacterium causes the infection. | Bacteria can pass in semen or vaginal fluid during vaginal, anal or oral sex without a condom, or on shared sex toys. | Correct condom use creates a barrier to infected fluids. Testing identifies infections that may otherwise be passed on; treatment of the infected person and partners, followed by avoiding sex until treatment is complete, prevents onward transfer. |
| HIV | A virus destroys white blood cells, weakening immune defence. Untreated HIV infection can progress to AIDS. | HIV can pass in infected semen, vaginal fluids or blood during sexual contact. It is not spread by ordinary contact such as hugging or sharing food. | Correct condom use reduces contact with infected fluids. Testing allows an infected person to receive treatment and take precautions. Effective HIV treatment can reduce the virus in the blood enough to prevent sexual transmission. |
HIV is the pathogen that is transmitted. AIDS is the advanced syndrome that can develop after HIV has severely weakened the immune system; AIDS is not a separate pathogen that is caught from another person.
5.8 · Choosing an effective prevention measure
The same barrier principle applies to both infections: reduce contact with fluids that may contain the pathogen. Condoms reduce the opportunity for infected semen, vaginal fluid or blood to reach another person's mucous membranes. Avoiding sexual contact removes this transmission route. Neither a contraceptive pill nor another method that only prevents pregnancy provides that physical barrier.
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