Viral Diseases

Viruses are the smallest type of pathogen and cannot reproduce on their own — they must invade a living host cell. This lesson covers the three viral diseases named on the AQA GCSE Biology specification: measles, HIV and tobacco mosaic virus (TMV). You need to know the symptoms, transmission, and treatment or prevention of each.

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How Viruses Cause Disease — A Recap

Viruses are not true living organisms. They are simply a strand of genetic material (DNA or RNA) surrounded by a protein coat (capsid). Because they have no cellular machinery of their own, they must:

1. Attach to a host cell and inject their genetic material
2. Use the host cell's enzymes and ribosomes to replicate their DNA/RNA and make new viral proteins
3. Assemble many new virus particles inside the host cell
4. The host cell bursts (lyses), releasing hundreds of new viruses that go on to infect more cells

This process of cell destruction is what causes the symptoms of viral disease.