OxfordAQA A Level Biology: The Causes of Disease — Revision Notes

Condensed for the final weeks. For the full explanation, use the Causes of Disease study guide.

Two mechanisms of pathogen damage (3.2.1.1)

Mechanism	What happens
Cell damage	The pathogen directly destroys host tissue
Toxin production	The pathogen releases chemicals that disrupt cell/tissue function, sometimes far from the infection site

Pathogens (bacteria, viruses) cause disease by penetrating one of a mammal’s interfaces with the environment, which include the digestive, reproductive and gas-exchange systems (this is not a complete list – the skin is a further interface). Always name the specific interface relevant to a given example — the gas-exchange system for an airborne respiratory pathogen, for instance — rather than describing entry in the abstract.

Lifestyle, CHD and cancer (3.2.1.2)

Specific risk factors are associated with coronary heart disease and cancer, and changes in lifestyle can reduce that risk. This sub-topic is a data-handling skill as much as a content one: practise reading incidence data (disease rates plotted against a risk factor such as smoking prevalence or dietary fat intake) and drawing a conclusion that explicitly separates correlation from causation.

Worked example: evaluating a risk-factor claim

Data show countries with higher average saturated fat intake have higher rates of coronary heart disease.

Observation: higher fat intake correlates with higher CHD rate in the data
Conclusion:  consistent with fat intake being a risk factor
Limitation:  correlation is not proof of causation -- other variables
             (smoking, activity levels, genetics) could also explain the
             difference, so a single factor cannot be confirmed as the
             sole cause from this data alone

Exam answers that state the limitation explicitly, rather than only describing the correlation, score in the top band.

Key terms

Pathogen — an agent of infection (bacteria, viruses) capable of causing disease. Toxin — a poisonous chemical a pathogen produces that disrupts cell or tissue function, sometimes distant from the infection site. Interface — a boundary with the environment (digestive, reproductive, gas-exchange) through which pathogens enter. Risk factor — a variable statistically associated with increased disease likelihood, without necessarily being proven to cause it. Correlation — a statistical relationship between two variables that alone does not establish causation.

Common mistakes

• Naming only cell damage as a mechanism and omitting toxin production, or vice versa.
• Failing to name a specific interface when explaining how a pathogen enters the body.
• Treating a correlation between a lifestyle factor and disease incidence as proof of causation.
• Discussing coronary heart disease and cancer as though they share identical risk factors, when the specification treats them separately.

Quick self-test

1. Name the two mechanisms by which a pathogen can cause disease.
2. Name three interfaces through which a pathogen can enter a mammal’s body.
3. Why does a correlation between a risk factor and disease incidence not prove causation?
4. Name one specific risk factor for coronary heart disease and one for cancer.
5. Give one worked example of a pathogen that causes disease predominantly through toxin production rather than cell damage.

Answers: 1. Cell damage and toxin production. 2. Digestive, reproductive and gas-exchange systems are the specification’s named examples (the skin is a further valid interface). 3. Because other variables that differ between the groups compared could also explain the difference in disease rate; establishing causation requires additional evidence such as a controlled study or an understood biological mechanism. 4. Any specification-consistent example, e.g. smoking or high dietary saturated fat for CHD; smoking or UV exposure for cancer. 5. The bacterium responsible for tetanus, which causes disease mainly via a toxin affecting the nervous system rather than through direct destruction of host tissue.

Naming a pathogen and its mechanism

A strong answer does more than state “pathogens cause disease” — it names a specific pathogen and identifies which mechanism, or both, it uses. For example, the bacterium responsible for tetanus causes disease almost entirely through toxin production (the toxin affects the nervous system, causing muscle spasms, while the bacterium itself does relatively little direct tissue damage), whereas many viruses cause disease predominantly through direct cell damage as they replicate inside and rupture host cells. Being able to give a worked example like this, rather than a purely general statement, is the difference between a mid-band and a top-band answer on this sub-topic.

Interpreting incidence data precisely

When a question presents disease-incidence data linked to a lifestyle risk factor, work through it in three explicit steps: first describe the pattern shown (for example, “as smoking prevalence increases across the countries shown, the incidence of the disease also increases”); second, state what conclusion the data is consistent with (smoking being a risk factor for that disease); third, name at least one other variable that could also explain the pattern (diet, genetics, age distribution, access to healthcare) before concluding that causation has not been proven by correlation alone. Skipping the third step is the single most common way marks are lost on this sub-topic, since examiners are specifically testing whether a candidate can evaluate, not just describe, the evidence presented.

How this connects across Unit 2

3.2.1 deliberately sits before the unit’s more system-specific sub-topics (digestion, cholera, HIV, blood defence, circulation) because it establishes the pathogen/lifestyle distinction those later sub-topics assume. When a later question on, for example, HIV or cholera asks how the pathogen causes harm, returning to the cell-damage/toxin-production framework from 3.2.1 is usually the fastest route to a precise answer, rather than treating each disease as an unrelated case study.

Related resources

• The Causes of Disease study guide
• The Causes of Disease practice questions

Official syllabus

OxfordAQA International AS and A-level Biology (9610) specification, Version 5.1 — oxfordaqa.com/9610.