How one disease can lead to anotherSpec 5.3
In short
One disease can lead to another by making a person more susceptible. HIV destroys white blood cells, so people with HIV are more likely to develop tuberculosis. A disease can also damage barriers such as the skin, cause poor nutrition or lead to poor mental health. A correlation in data does not prove that one disease causes another.
Having one disease can make you more susceptible (more likely to catch) other diseases. There are several ways this happens.
- A weakened immune system. HIV destroys white blood cells, so the body cannot fight off other pathogens. People with HIV are much more likely to develop infections such as tuberculosis.
- Damaged barriers. A disease that damages the skin or the lining of the airways makes it easier for other pathogens to get in.
- Poor nutrition. Some diseases make it hard to eat or absorb food. A poorly nourished body makes fewer of the cells and chemicals needed for defence.
- Mental health. A long-term physical illness can lead to depression or anxiety. Poor mental health can in turn make physical health worse.
Using data
You may be given a table, graph or scatter diagram linking two diseases. A scatter diagram can show a correlation: as one variable changes, the other changes too. A correlation does not prove that one disease causes the other. Something else could be responsible for both.
Data from a larger, randomly chosen sample is more reliable than data from a small one. Also check that the people sampled are representative of the group the conclusion is about.
Reading a scatter diagram
A scatter diagram shows that in 20 regions, the regions with more people infected with HIV also have more people with tuberculosis. Describe the pattern and state what can be concluded.
- The points slope upwards from left to right.
- As HIV infections increase, tuberculosis cases increase, so there is a positive correlation.
- A correlation does not show cause, but it is consistent with HIV making people more susceptible to tuberculosis.
Answer: Positive correlation between HIV and tuberculosis. It supports, but does not prove, that HIV increases susceptibility to tuberculosis.
(opens full size in a new tab)Quick check
Give the WHO definition of health.
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A state of complete physical, mental and social well-being and not merely the absence of disease or infirmity.
What is the main difference between communicable and non-communicable diseases?
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Communicable diseases are caused by pathogens and can be passed on; non-communicable diseases cannot be passed on.
Explain why someone with HIV is more likely to get tuberculosis.
Show answer
HIV destroys white blood cells, so the immune system is weakened and cannot fight off other pathogens.
What does a positive correlation on a scatter diagram show?
Show answer
As one variable increases, the other also increases.
Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026