Communicable (infectious) diseases: Viruses live and reproduce inside cells, causing cell damage.

The idea

Viruses are not cells and cannot carry out life processes independently, so they are obligate parasites. A virus particle attaches to a host cell, injects its genetic material, and uses the host's ribosomes and enzymes to make new virus particles. Because replication happens inside the cell, the host cell's normal chemistry is disrupted and the cell may burst, releasing many new viruses. This explains why viral infections damage tissues and cause symptoms. For example, influenza virus infects cells lining the airways; the resulting cell damage contributes to fever and coughing. Antibiotics do not kill viruses because viruses lack bacterial targets such as cell walls.

What earns the marks

• Viruses are not cells and are much smaller than bacteria.
• Viruses reproduce only inside living host cells, using the host cell's machinery.
• Viral genetic material enters the host cell and directs synthesis of new virus particles.
• Replication inside the cell disrupts normal cell activity and damages or destroys the cell.
• Cell damage and the release of new virus particles produce the symptoms of viral disease.
• Antibiotics are ineffective against viruses because viruses do not have bacterial cell structures.

Common mistakes and how to put them right

• Saying viruses are a type of bacterium.

  Put it right by stating viruses are non-cellular and much smaller, with a different structure.

• Claiming viruses reproduce on their own.

  Put it right by explaining they need a living host cell and its enzymes and ribosomes.

• Confusing 'live' with independent life.

  Put it right by describing viruses as obligate parasites that can only replicate inside cells.

Exam tips

• Use the phrase 'inside cells' when explaining viral reproduction, because the specification links reproduction to cell damage.
• Link each stage to the consequence for the host cell, for example entry, replication, then cell damage.
• If asked why antibiotics do not work, compare viruses with bacteria rather than simply saying 'viruses are different'.