Viruses living in cells can be the trigger for cancers. • Immune reactions…

The idea

Two indirect health effects are described. First, some viruses insert genetic material into host cells and disrupt normal control of cell division, so cells divide uncontrollably and a tumour forms; examples include human papillomavirus linked to cervical cancer and hepatitis B linked to liver cancer. Second, an immune reaction that begins as a defence against a pathogen can become misdirected or exaggerated, producing allergy symptoms.

For example: An infection may prime immune cells so that later exposure to a harmless allergen triggers histamine release, causing a skin rash, or narrowing of the airways in asthma.

What earns the marks

• Some viruses enter host cells and alter the control of the cell cycle, causing uncontrolled cell division and tumour formation.
• A named example strengthens the answer, such as human papillomavirus and cervical cancer or hepatitis B and liver cancer.
• An immune reaction first caused by a pathogen can later be triggered by a harmless substance, producing an allergic response.
• Allergy symptoms arise from immune chemicals such as histamine, which cause effects such as skin rashes, swelling or airway narrowing in asthma.
• The pathogen is the initial trigger; the cancer or allergy is a consequence, so the causal sequence must be stated clearly.
• The two clauses are separate mechanisms and should be explained independently rather than merged into one process.

Common mistakes and how to put them right

• Stating that a virus is a cancer cell.

  Put it right by saying the virus triggers changes in host cells that lead to uncontrolled division.

• Saying allergies are caused directly by the pathogen rather than by an immune reaction that the pathogen initially triggered.

• Confusing the roles of histamine and antibodies.

  Put it right by describing histamine as a chemical released during an allergic response.

• Treating all viruses as carcinogenic.

  Put it right by specifying that only some viruses are linked to particular cancers.

Exam tips

• Use the wording 'trigger' to show that the virus or pathogen starts a process rather than being the disease itself.
• Give one named virus–cancer link and one named allergic condition to demonstrate breadth.
• For allergy questions, sequence the answer: pathogen exposure → immune sensitisation → allergen exposure → histamine release → symptoms.
• Keep the two bullet points separate in your answer so each mechanism is clearly credited.