Health issues: Viruses living in cells can be the trigger for cancers.

The idea

Viruses are obligate intracellular parasites: they must enter a host cell and use its machinery to replicate. Some viruses insert their genetic material into the host DNA, and this can disrupt the normal control of the cell cycle. Genes that normally regulate division, such as tumour suppressor genes and proto-oncogenes, may be altered so the cell divides repeatedly without restraint, forming a tumour. A tumour that invades and spreads is a cancer. For example, human papillomavirus (HPV) infects cells of the cervix and is linked to cervical cancer; vaccination reduces this risk. Not every viral infection causes cancer, because further mutations and other factors are usually needed. Understanding this link explains why some cancers can be prevented by vaccination and why early detection matters.

What earns the marks

• Viruses are obligate intracellular parasites and must live inside host cells to replicate.
• Viral genetic material can be inserted into the host cell DNA, altering normal gene function.
• Changes to genes controlling the cell cycle, such as tumour suppressor genes and proto-oncogenes, can lead to uncontrolled cell division.
• Uncontrolled cell division produces a tumour; a malignant tumour that invades and spreads is a cancer.
• A named example is human papillomavirus (HPV) infecting cervical cells and being linked to cervical cancer.
• Not all viral infections cause cancer; additional mutations or risk factors are usually involved.

Common mistakes and how to put them right

• Thinking viruses are cells.

  Put it right by stating viruses are much smaller than cells, are not cells, and must reproduce inside a host cell.

• Believing a virus directly 'eats' or destroys tissue to form a cancer.

  Put it right by explaining that cancer arises from altered control of the host cell cycle, not from direct digestion.

• Assuming every infection with a cancer-linked virus causes cancer.

  Put it right by noting that cancer usually requires further genetic changes and other risk factors.

Exam tips

• Use the phrase 'uncontrolled cell division' when explaining how a tumour forms, and link it to a change in the host cell's genes.
• Give one named example, such as HPV and cervical cancer, to show the link is real rather than theoretical.
• If asked to evaluate, mention that the link is a risk factor, not a certainty, and refer to prevention such as vaccination.