Lesson 2 of 5 / Infection, vaccination and disease spread
Tuberculosis: transmission and infection
Why does inhalation matter more than simply touching an infected person?
I can explain airborne transmission and infection by Mycobacterium tuberculosis.
I can explain transmission and infection by Mycobacterium tuberculosis.Syllabus
Syllabus 9477, Extension A(h). Explain transmission and infection by Mycobacterium tuberculosis.
About 7 min
Make a guess. You are not marked.
Someone has latent TB infection. Are they spreading TB every time they cough?
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Latent infection is not usually infectious.
Only active pulmonary or laryngeal disease produces infectious airborne particles.
The key idea
Airborne particles can deliver M. tuberculosis to the lungs; infection and active, transmissible disease are distinct states.
Connect cause and consequence
Where can containment change the outcome?
Airborne particles from an infectious pulmonary/laryngeal case can be inhaled and reach alveoli. Exposure does not guarantee established infection.
Brown rods = bacteria; green inner cell = macrophage; blue ring = organised immune-cell containment. The ring summarises a granulomatous response, not its complete histology. Selecting a later state does not imply all infections inevitably follow it.
Explanation
Mycobacterium tuberculosis is a bacterial pathogen. Infectious airborne particles can be generated by people with infectious pulmonary or laryngeal disease and inhaled by others. Ventilation, exposure duration and particle concentration influence transmission opportunities; ordinary surface contact is not the main route.
Inhaled bacteria can reach the alveoli and be taken up by macrophages. The pathogen can resist normal intracellular killing and persist or multiply. Immune cells may organise around infected material, forming granulomatous structures that can contain infection without necessarily eliminating every bacterium.
A person can have latent infection without active disease or ordinary infectiousness. If control fails, including when immunity is weakened, active disease can develop and damage lung tissue; disease can also involve other sites. Do not assume every infected person is equally infectious at every stage.
Keep three stages separate: exposure to airborne bacteria, establishment of infection, and development of disease. Evidence of one stage does not prove every later stage. This distinction helps explain why public-health control addresses both transmission conditions and the host-pathogen interaction.
Step by step
- 1
Trace the route
Source, airborne particles, inhalation and alveoli.
- 2
Trace the cellular interaction
Macrophage uptake may not eliminate the bacterium.
- 3
Separate infection and disease
Persistence can occur without active symptoms or transmission.
Worked example
Work through the evidence
Why can an infected person remain well for years and later develop disease when immunity weakens?
One way to explain it
Immune containment may hold persistent bacteria in check. Reduced control can allow renewed multiplication and tissue damage, leading to active disease.
Why this answer works
- Persistence is not the same as complete clearance.
- Latent infection is not equivalent to continuous active disease.
Watch out for this
A student says: "Anyone with latent TB infection continuously spreads TB by coughing." What is wrong with this?
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Latent infection is distinct from infectious active pulmonary/laryngeal disease.
Check your understanding
Which is the principal transmission route described for pulmonary TB?
Show the answer
Inhalation of infectious airborne particles
This route can deliver bacteria to the lungs.
Quick recall
Why does inhalation matter more than simply touching an infected person?