Health issues: Different types of disease may interact.

The idea

Diseases rarely occur in isolation. A communicable disease can weaken the body so that a non-communicable disease becomes more likely, and a non-communicable disease can make a communicable infection more serious. For example, infection with HIV destroys lymphocytes, so the immune system cannot control other pathogens and the person develops AIDS, in which infections such as tuberculosis become life-threatening. Conversely, a person with type 2 diabetes may heal slowly and be more prone to skin infections. Another interaction is that a pathogen may trigger an immune response that damages healthy tissue, as in some cases of rheumatic fever. Understanding these links helps explain why vaccination, good nutrition and treating one condition can reduce the risk from another.

What earns the marks

• Communicable diseases are caused by pathogens and can be spread between organisms, whereas non-communicable diseases are not transmitted and include conditions such as type 2 diabetes and coronary heart disease.
• A communicable disease can increase the risk of a non-communicable disease, for example a viral infection may trigger certain cancers or long-term organ damage.
• A non-communicable disease can increase the risk of a communicable disease, for example diabetes can lead to poor wound healing and increased susceptibility to infection.
• A communicable disease can make another communicable disease more severe, for example HIV destroys lymphocytes so the body cannot fight off infections such as tuberculosis.
• Interactions can be described using specific named examples, and the direction of the effect should be stated clearly rather than simply listing two diseases.

Common mistakes and how to put them right

• Thinking that all diseases are infectious.

  Put it right: Non-communicable diseases such as asthma and type 2 diabetes are not caused by pathogens and cannot be passed from person to person.

• Assuming that if two diseases occur together they must directly cause each other.

  Put it right: An interaction must be explained, for example by stating how one condition weakens defences or damages an organ, rather than just naming two conditions.

• Confusing correlation with causation.

  Put it right: A statistical link between two diseases does not prove that one causes the other; a biological mechanism should be described.

Exam tips

• Use the command word in the question: if asked to explain, give a mechanism linking the two diseases rather than just naming them.
• Choose one clear example, such as HIV and tuberculosis, and state the effect of HIV on lymphocytes and the consequence for other infections.
• Link your answer to the idea of health as a state of physical and mental well-being, so that interactions are placed in the wider context of disease risk.