16.6 Sexually transmitted infections

2026 Syllabus Objectives

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

What are Sexually Transmitted Infections? 🦠

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A sexually transmitted infection (STI) is an infection that is transmitted through sexual contact. These infections are passed from one person to another during sexual intercourse when body fluids from infected individuals come into contact with the body fluids of uninfected individuals.

STIs can be caused by various pathogens including bacteria, viruses, and other microorganisms. One of the most significant viral STIs is caused by the human immunodeficiency virus (HIV).

HIV as a Pathogen 🔬

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Key Facts about HIV

Human immunodeficiency virus (HIV) is a pathogen that causes a sexually transmitted infection. HIV is a particularly dangerous virus because it targets the body's immune system itself.

How HIV Affects the Body

HIV infects white blood cells, particularly a type called T cells. These T cells are normally the body's strongest defence against viruses. The virus has the following effects on the immune system:

  • Targets T cells specifically: HIV infects and destroys T cells over time
  • Weakens immune defence: Over a long period, HIV slowly destroys T cells
  • Difficult to fight: Because HIV attacks the very cells which would normally kill viruses, it is very difficult for someone's own immune system to protect them against HIV
  • Progressive destruction: Several years after infection with the virus, the numbers of certain kinds of T cells are so low that they are unable to fight against other pathogens effectively

Important: A person infected with HIV is said to be HIV positive.

HIV Infection Leading to AIDS ⚠️

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From HIV to AIDS

HIV infection may lead to AIDS. AIDS stands for Acquired Immune Deficiency Syndrome.

Timeline and Progression

  • About ten years after initial infection with HIV, an untreated person is likely to develop symptoms of AIDS
  • During this time, the immune system becomes progressively weaker as T cells are destroyed

Symptoms and Effects of AIDS

When a person develops AIDS, they become very vulnerable to other infections because their immune system can no longer function properly:

  • Opportunistic infections: They may develop infections such as pneumonia that healthy immune systems would normally fight off
  • Cancer development: They may develop cancer, because one function of the immune system is to destroy body cells that may be beginning to produce cancers
  • Brain damage: Brain cells are also quite often damaged by HIV
  • Multiple illnesses: A person with AIDS usually dies from a collection of several illnesses

Modern Treatment

Today, there are excellent drugs to control HIV, called antiretrovirals. These drugs:

  • Stop the virus from multiplying inside the person's cells
  • Allow HIV-positive people to live healthy and normal lives when taken regularly
  • Greatly reduce the progression from HIV to AIDS

Researchers are working on the production of a vaccine against HIV infection, and there have been promising signs in early trials.

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