Tuberculosis (TB) is a communicable disease caused by the bacterium Mycobacterium tuberculosis. It is spread by droplets released into the air when an infected person coughs or sneezes. Explain how TB spreads from one person to another and how the body's defences attempt to prevent infection once the bacteria have been inhaled.

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).

Tuberculosis (TB) is one of the world's leading infectious diseases. In crowded, poorly ventilated environments the disease spreads rapidly. Despite this, many people who inhale the bacterium do not develop symptoms.

Model answer (5 marks)

TB is spread when an infected person coughs or sneezes, releasing droplets containing Mycobacterium tuberculosis into the air.

When these droplets are inhaled, mucus in the airways traps the bacteria before they can reach the lungs.

The cilia in the respiratory tract move the mucus, and any trapped bacteria, upward towards the throat where it can be swallowed or expelled.

If swallowed, hydrochloric acid in the stomach destroys the bacteria.

If the bacteria reach the lungs, white blood cells (phagocytes) engulf and destroy them by phagocytosis.

Examiner tips

  • Use the exact sequence of defence steps: mucus, cilia, stomach acid, phagocytosis.
  • Show the cause (cough/sneeze) and effect (droplets) clearly.
  • Use the terminology ‘phagocytes’ and ‘phagocytosis’.
  • Keep the answer concise – 5 points, not more than 5 sentences.

Common mistakes

  • Mixing up the order of defence mechanisms (e.g., stating phagocytosis before mucus trapping).
  • Using vague terms like ‘immune system’ instead of specific cells or actions.
  • Omitting the role of cilia or stomach acid.

Mark scheme (5 marks)

  1. TB is spread when an infected person coughs or sneezes, releasing droplets containing Mycobacterium tuberculosis into the air
  2. Mucus in the airways traps the bacteria / pathogens before they can reach the lungs
  3. Cilia waft the mucus (and trapped bacteria) upward towards the throat so it can be swallowed or expelled
  4. Hydrochloric acid in the stomach destroys any bacteria that are swallowed
  5. If bacteria reach the lungs, white blood cells / phagocytes can engulf and destroy them by phagocytosis

Key terms in this question

communicable disease

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