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Pneumocystis carinii colonises some healthy people harmlessly but causes lethal pneumonia when immunity is weakened. How is its role in that pneumonia classified?
AA sufficient cause on its own
BA necessary but not sufficient cause
CA component of no sufficient cause
DA determinant of the environment
Answer & Solution
Correct answer: B. A necessary but not sufficient cause
1. In the causal pie model, each slice is a component cause and a complete pie is a sufficient cause.
2. A component that appears in every pie is a necessary cause, because disease cannot occur without it.
3. The organism must be present for pneumocystis pneumonia, so it is necessary.
4. Its presence alone does not produce disease in healthy people, so it is not sufficient.
5. Calling it a sufficient cause ignores the healthy carriers who never fall ill.
6. Calling it a component of no sufficient cause contradicts the fact that the pneumonia does occur.
_Source: CDC Principles of Epidemiology in Public Health Practice, 3rd Edition, section Component Causes and Causal Pies_
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