One Disease Can Increase the Risk of Another
Definition
Immune system
The body's network of cells, tissues and organs that recognises and destroys pathogens and the abnormal cells they produce.
Definition
Susceptibility
The likelihood that a person will develop a disease after exposure to its cause.
- The immune system protects the body by recognising and destroying pathogens and infected cells.
- Susceptibility is the likelihood that a person will develop a disease after exposure to its cause.
- A first disease can damage a defence barrier, weaken immune responses or alter body tissues, which increases susceptibility to a second disease.
- A causal explanation must identify the first disease, the biological change it causes and why that change increases the second disease.
Key Idea
- A complete explanation links the first disease to a biological change and then links that change to the increased chance of a second disease.
HIV Weakens Immune Defence
- HIV infects and destroys particular white blood cells that coordinate immune responses.
- As the number of these cells falls, the body produces a weaker response to pathogens.
- Pathogens that would normally be controlled can reproduce and cause opportunistic infections.
- Untreated HIV infection can progress to AIDS, when immune function is severely damaged.
- HIV binds to specific white blood cells and uses them to reproduce, progressively reducing the cells available to coordinate immune responses.
- With fewer effective lymphocytes, antibody production and activation of other immune cells become less effective.
- Opportunistic pathogens then reproduce to damaging numbers even when they would usually be controlled.
Example
- HIV reduces the number of white blood cells, so fewer pathogens are destroyed and the person becomes more susceptible to infections such as tuberculosis.
Barrier Damage Creates Entry Routes
- Intact skin forms a physical barrier that prevents pathogens entering body tissues.
- A burn, wound or skin disease breaks this barrier and exposes tissue beneath it.
- Bacteria can then enter, reproduce and cause an infection.
Common Mistake
- Do not write that a wound creates bacteria, because the wound provides an entry route for bacteria already in the surroundings.
Infections Can Trigger Other Diseases
- Some viruses alter the control of cell division, which can increase the risk of certain cancers.
- For example, persistent infection with some strains of human papillomavirus increases the risk of cervical cancer.
- A severe physical disease can also reduce activity, disrupt sleep and cause long-term stress, which can worsen mental health.
- Some infections increase cancer risk because viral genetic material or long-term inflammation can disrupt control of cell division.
Exam technique
- Use a causal chain in an explanation: name the first disease, state what it damages or changes, then state why that raises susceptibility to the second disease.
- The link 'weaker immune system, so fewer pathogens are destroyed' is creditworthy because it gives both mechanism and consequence.
Separate Cause From Correlation
- If two diseases occur together, this does not by itself prove that one caused the other.
- Evidence for causation must show a plausible biological mechanism and rule out other factors that could explain the association.
Self review
- Define susceptibility.
- Explain how HIV increases susceptibility to other infections.
- Explain how a burn can lead to bacterial infection.
- Why does an association between two diseases not always prove causation?
