Chlamydia is a communicable disease caused by the bacterium Chlamydia. Explain how the body's defences and the nature of Chlamydia's transmission make it difficult to prevent the spread of this infection.

Pearson Edexcel A-Level Biology A (9BI0) — 5.2 Pathogens and infection · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Chlamydia is one of the most commonly reported sexually transmitted infections in the UK. It is spread through sexual contact and often produces no symptoms in infected individuals, meaning many people are unaware they carry the bacterium.

Model answer (5 marks)

Chlamydia is a communicable disease spread through direct sexual contact, involving transfer of infected bodily fluids.

Many infected people show no symptoms, so they do not seek treatment and unknowingly continue to spread the bacterium.

The immune system produces antibodies against Chlamydia antigens, but this response is slow and may not prevent transmission to a new host.

Physical barriers such as skin act as a first line of defence, but sexual contact bypasses these barriers, allowing direct transmission of the pathogen.

If left untreated, Chlamydia can lead to infertility; therefore early detection and antibiotic treatment are essential to prevent long‑term harm and further spread.

Examiner tips

  • Use the exact phrase "direct sexual contact" to show understanding of transmission.
  • Mention both asymptomatic carriers and the delayed antibody response to cover all points.
  • Include the consequence of infertility to demonstrate knowledge of disease impact.
  • Keep each point concise – 1–2 sentences per mark.

Common mistakes

  • Confusing Chlamydia with other STIs; use the correct bacterium name.
  • Omitting the asymptomatic nature of the infection.
  • Failing to link the immune response to the difficulty of prevention.

Mark scheme (5 marks)

  1. Chlamydia is a communicable disease spread through direct sexual contact / transfer of infected bodily fluids
  2. Many infected individuals have no symptoms, so they do not seek treatment and unknowingly continue to spread the bacterium
  3. The immune system produces antibodies specific to Chlamydia antigens, but this response may not occur fast enough to prevent transmission to others
  4. Physical barriers such as skin act as a first line of defence, but sexual contact bypasses these barriers allowing direct transmission
  5. If left untreated, Chlamydia can lead to infertility, so early detection / use of antibiotics is important to prevent long-term harm and further spread

Key terms in this question

communicable disease · Chlamydia

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