Biology (0610) | Sexually transmitted infections

Welcome to the study notes for the chapter on Sexually Transmitted Infections (STIs)!

This is a short but incredibly important section of your Reproduction topic. Understanding how these infections are spread and, crucially, how they are prevented and controlled is vital not just for your exams, but for real-world health and awareness.
We will keep the biological concepts clear and straightforward. Let’s dive in!

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1. Defining Sexually Transmitted Infections (STIs)

What exactly is an STI?

An STI, or Sexually Transmitted Infection, is exactly what the name suggests: it is an infection caused by a pathogen (a disease-causing organism) that is transmitted from one person (host) to another primarily through sexual contact.

Remember: This transmission happens when body fluids are exchanged during sexual activity.

Quick Review Box: STI Definition

• What: An infection caused by a pathogen.
• How: Transmitted through sexual contact.

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2. The Case Study: HIV and AIDS

The IGCSE syllabus focuses specifically on the most well-known STI: the one caused by the Human Immunodeficiency Virus.

a) The Pathogen: HIV

The pathogen responsible for this STI is the Human Immunodeficiency Virus (HIV).

• HIV is a virus, meaning it needs a host cell to reproduce.
• It specifically targets and destroys cells in the body's immune system (the white blood cells, specifically T-helper cells, which coordinate the body's defense).

Analogy Alert! Think of your immune system as a country's army. HIV is a spy that infiltrates and destroys the generals (the T-helper cells). Once the generals are gone, the army can no longer organize a proper defense.

b) The Result: AIDS

If an HIV infection is left untreated, the damage to the immune system eventually becomes severe. This leads to the condition known as Acquired Immunodeficiency Syndrome (AIDS).

• A person with AIDS has a severely weakened immune system.
• They cannot fight off infections that a healthy person would easily manage (these are called opportunistic infections, like certain types of pneumonia or cancer).

Important Distinction:
HIV is the virus that causes the infection.
AIDS is the syndrome (the collection of symptoms/diseases) that results from the immune system failure caused by long-term HIV infection.

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3. Transmission of HIV

Understanding how HIV is passed on is crucial for effective control. HIV is present in high concentrations in certain body fluids.

How HIV is transmitted (Core Focus)

HIV is transmitted when infected bodily fluids enter the bloodstream or mucous membranes (the moist linings of the body, like the mouth or genitals) of another person. The four primary methods of transmission are:

1. Unprotected Sexual Contact: Through the exchange of semen, pre-ejaculate, and vaginal/rectal fluids during sex. This is the most common route globally.
2. Blood-to-Blood Contact: Sharing needles (e.g., among drug users) or using unsterilised equipment for things like tattooing or piercing. (Note: Modern blood transfusions are tested and safe, but this was a major risk historically.)
3. Mother to Child (During Pregnancy/Birth): The virus can pass from the mother's blood to the baby across the placenta.
4. Mother to Child (Through Breast Milk): The virus can be transmitted during breastfeeding.

Did You Know?

HIV cannot survive for long outside the human body. You absolutely cannot get HIV from hugging, shaking hands, sharing cutlery, coughing, sneezing, or using the same toilet seat.

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4. Controlling the Spread of STIs

Controlling the spread of STIs (including HIV) requires a combination of behavioral changes, medical strategies, and public health measures. These controls aim to break the chain of transmission.

Methods used to Control the Spread of STIs (including HIV)

1. Education and Awareness

This is the first line of defense. People must be taught:

• The importance of knowing how STIs are transmitted.
• The risks associated with having multiple sexual partners.
• How to use protective measures correctly.

2. Barrier Methods (e.g., Condoms)

Using a barrier method (like a condom) during sexual intercourse creates a physical barrier that prevents the exchange of infected body fluids (semen, vaginal fluids). This greatly reduces the risk of transmission.

3. Screening and Testing

Regular testing allows infected individuals to be identified early, even if they show no symptoms. Once identified, they can be treated (if possible) and counselled on preventing further transmission.

4. Contact Tracing and Treatment

When an infection is confirmed:

• The patient receives appropriate antibiotics (for bacterial STIs like Gonorrhoea) or antiviral medication (for viral STIs like HIV, though this doesn't cure it, it controls the virus).
• Contact tracing occurs: health services confidentially identify and test recent sexual partners of the infected person.

5. Prevention of Mother-to-Child Transmission (PMTCT)

In HIV prevention, specific measures are taken:

• Pregnant HIV-positive women are given antiviral drugs to drastically reduce the viral load in their blood.
• This treatment continues during labour and is often given to the newborn baby for a short period.
• In affected areas, mothers may be advised to feed the baby formula instead of breast milk to prevent transmission via breastfeeding (though this decision depends on access to clean water and formula).