# Explain how viruses cause disease in an infected host, with reference to both direct and indirect mechanisms of cell damage.

> IB DP Biology Higher Level (2023 syllabus) — A2.3 Viruses (HL only) · Explain · 4 marks

## Mark scheme (4 marks)

1. Viruses cause direct cell damage by lysing host cells during the lytic cycle / by rupturing the host cell membrane upon release of new virions.
2. 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.
3. 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.
4. 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

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
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