Explain why the presence of one disease can lead to a higher…

The idea

One disease can make a host more vulnerable to others through several linked mechanisms. A communicable disease may damage tissues or defences: for example, a virus infecting cells of the respiratory tract can damage the lining, allowing bacteria to invade more easily. Some pathogens weaken the immune system, so the body responds less effectively to a second infection. A non-communicable disease can also raise susceptibility: poorly controlled diabetes can impair healing and immune responses, and lifestyle factors such as poor diet can reduce resistance. In each case the first disease reduces the body's barriers or immune capacity, so a second disease is more likely to establish. The explanation must link the first disease to a reduced defence and then to increased likelihood of the second.

What earns the marks

• The first disease may damage physical barriers such as skin, mucous membranes or the respiratory lining, allowing a second pathogen to enter.
• The first disease may weaken the immune system so the body responds less effectively to a second infection.
• A non-communicable disease such as diabetes can impair healing and immune responses, raising susceptibility.
• Lifestyle factors linked to disease, such as poor diet, can reduce resistance to infection.
• A clear chain of reasoning: first disease reduces defences, therefore a second disease is more likely to establish.

Common mistakes and how to put them right

• Saying the second disease is caused directly by the first.

  Put it right: The first disease reduces defences, making infection by a separate pathogen more likely.

• Ignoring non-communicable diseases: correction — conditions such as diabetes can also increase susceptibility by impairing immune responses and healing.

• Giving only a named example without explaining the mechanism.

  Put it right: Link the first disease to a damaged barrier or weakened immune system, then to increased likelihood of the second.

Exam tips

• Build a chain: first disease → reduced defence → higher chance of second disease.
• Use a specific mechanism such as damage to the respiratory lining or a weakened immune system.
• Include both communicable and non-communicable examples to show breadth.