Cholera is a communicable disease caused by the bacterium Vibrio cholerae. Explain how Vibrio cholerae spreads between individuals and describe TWO ways in which the human body can defend itself against this pathogen before it enters the bloodstream.

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).

Cholera outbreaks are most common in areas where clean drinking water and adequate sanitation are not available. The bacterium Vibrio cholerae produces toxins that cause severe diarrhoea, leading to dehydration.

Model answer (5 marks)

Vibrio cholerae is spread by water‑borne transmission – it is carried in contaminated drinking water or food that has been washed with polluted water.

Before reaching the bloodstream the body can:
1. The mucus lining the respiratory tract traps bacteria; the cilia beat to move the mucus and trapped microbes up toward the throat where they are swallowed or expelled.
2. The acidic gastric juice (hydrochloric acid) in the stomach destroys many bacteria that are swallowed.
3. The skin and cell walls act as a physical barrier preventing the pathogen from entering the bloodstream.

Examiner tips

  • Use the exact terms ‘water‑borne transmission’, ‘mucus’, ‘cilia’, ‘hydrochloric acid’, and ‘physical barrier’ – these are the words the mark scheme expects.
  • Show the sequence: contamination → ingestion → mucus/cilia or stomach acid as first line of defence.
  • Keep the answer concise – 5 marks, so one sentence per point.
  • Mention both mucosal and chemical barriers, not just one.

Common mistakes

  • Confusing the respiratory tract with the gastrointestinal tract – the question refers to ingestion, so focus on stomach acid and skin barriers.
  • Omitting the word ‘water‑borne’ or describing only food contamination – both are acceptable but ‘water‑borne’ is the key phrase.
  • Using vague terms like ‘immune system’ instead of the specific barriers listed in the mark scheme.

Mark scheme (5 marks)

  1. Vibrio cholerae is spread through contaminated water (water-borne transmission) / ingestion of food or water contaminated with the bacterium
  2. Mucus in the trachea / airways traps pathogens / bacteria before they can enter the body further
  3. Cilia waft / sweep the mucus (and trapped bacteria) away from the lungs / up towards the throat so it can be swallowed or expelled
  4. Hydrochloric acid in the stomach kills / destroys many pathogens / bacteria that are swallowed
  5. The skin / cell wall acts as a physical barrier preventing the pathogen from entering the bloodstream

Key terms in this question

communicable disease · pathogen · Vibrio cholerae

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