Explain how viruses can be transmitted between hosts, with reference to at least two distinct transmission routes.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Viruses are transmitted when viral particles reach a susceptible host and enter cells.
1. Direct contact – touching, kissing or sexual intercourse can transfer virus‑laden body fluids or skin surfaces to another person.
2. Airborne/droplet – coughing, sneezing or breathing releases respiratory droplets or aerosols that are inhaled by a new host.
3. Vector‑borne or indirect – arthropods such as mosquitoes can carry virus from one host to another, or contaminated fomites, water or food can transfer virus without direct contact.
4. In all routes the virus must attach to a cell receptor and deliver its genome into the cell for replication to occur.
1. Direct contact – touching, kissing or sexual intercourse can transfer virus‑laden body fluids or skin surfaces to another person.
2. Airborne/droplet – coughing, sneezing or breathing releases respiratory droplets or aerosols that are inhaled by a new host.
3. Vector‑borne or indirect – arthropods such as mosquitoes can carry virus from one host to another, or contaminated fomites, water or food can transfer virus without direct contact.
4. In all routes the virus must attach to a cell receptor and deliver its genome into the cell for replication to occur.
Examiner tips
- Use the word ‘transmission’ and give at least two routes; mention the need for receptor attachment and a susceptible host for full marks.
- Show clear separation of routes with bullet points or numbering to demonstrate understanding.
Common mistakes
- Confusing transmission with infection – students write about symptoms instead of routes.
- Omitting the requirement that the virus must successfully enter host cells via receptor attachment.
Mark scheme (4 marks)
- Direct contact transmission: viruses can be passed from one host to another through physical contact, such as touching, kissing, or sexual intercourse, allowing viral particles on body surfaces or in secretions to enter a new host.
- Airborne or droplet transmission: respiratory droplets or aerosols expelled during coughing, sneezing, or breathing carry viral particles that can be inhaled by a new host.
- Vector-borne or indirect transmission: some viruses are transmitted via biological vectors (e.g. arthropods such as mosquitoes) or via contaminated fomites / water / food, transferring viral particles to a new host without direct host-to-host contact.
- Viral particles must successfully enter host cells via receptor-mediated attachment; transmission is only effective if sufficient viral load reaches susceptible (non-immune) cells in the new host.
Key terms in this question
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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