Explain how viruses cause disease in an infected host, with reference to both direct and indirect mechanisms of cell damage.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Viruses cause disease by damaging host cells in two ways.
1. Direct damage – during the lytic cycle the virus replicates inside a cell, uses the cell’s ribosomes, enzymes and energy to make new virions, and then lyses or ruptures the cell membrane to release them, destroying the cell.
2. Indirect damage – the immune response to infected cells causes tissue injury. Inflammation, cytokine release and fever damage host tissues, and cytotoxic T lymphocytes recognise viral antigens presented on MHC I molecules and kill the infected cells, further reducing healthy cell numbers.
1. Direct damage – during the lytic cycle the virus replicates inside a cell, uses the cell’s ribosomes, enzymes and energy to make new virions, and then lyses or ruptures the cell membrane to release them, destroying the cell.
2. Indirect damage – the immune response to infected cells causes tissue injury. Inflammation, cytokine release and fever damage host tissues, and cytotoxic T lymphocytes recognise viral antigens presented on MHC I molecules and kill the infected cells, further reducing healthy cell numbers.
Examiner tips
- Use the word ‘direct’ and ‘indirect’ clearly; mention lytic cycle and cell lysis for direct damage.
- Show that indirect damage is immune‑mediated – inflammation, cytokines, fever and CTL killing – and link it to loss of healthy cells.
Common mistakes
- Confusing viral replication with cell lysis; forgetting to mention the lytic cycle. Failing to recognise that CTL killing is an indirect mechanism of damage.
Mark scheme (4 marks)
- Viruses cause direct cell damage by lysing host cells during the lytic cycle / by rupturing the host cell membrane upon release of new virions.
- Viruses can also cause direct damage by diverting host cell metabolism / hijacking host ribosomes, energy, and raw materials for viral replication, impairing normal cell function.
- Indirect damage occurs through the host immune response: inflammation / cytokine release / fever can damage host tissues even in the absence of direct viral infection of those tissues.
- Virally infected cells displaying viral antigens on MHC molecules are targeted and destroyed by cytotoxic T lymphocytes / T killer cells, contributing to loss of host cells.
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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