OxfordAQA A-Level Biology: The Causes of Disease (9610)

This guide covers 3.2.1 The Causes of Disease: Pathogens, Lifestyle and Genes, the opening sub-topic of Unit 2 (Biological Systems and Disease) in OxfordAQA International AS and A-level Biology (9610), Version 5.1, International AS exams May/June 2017 onwards, International A-level exams May/June 2018 onwards.

Where this fits in Unit 2

Unit 2 studies how organisms exchange substances with their environment and how disease can disrupt these systems. 3.2.1 sets up the unit’s central distinction — between disease caused by pathogens and disease linked to lifestyle and genetic factors — before the unit moves on to specific systems (digestion, cholera, HIV, blood defence, circulation).

Syllabus coverage

OXFORDAQA INTERNATIONAL AS AND A-LEVEL BIOLOGY (9610) — 3.2.1 THE CAUSES OF DISEASE: PATHOGENS, LIFESTYLE AND GENES

• 3.2.1.1 Pathogens — pathogens are agents of infection and include bacteria and viruses; disease can result when pathogens penetrate any of an organism’s interfaces with the environment (in a mammal these include the digestive, reproductive and gas-exchange systems); pathogens cause disease by damaging the cells of the host and by producing toxins
• 3.2.1.2 Lifestyle, coronary heart disease and cancer — specific risk factors are associated with coronary heart disease and cancer; changes in lifestyle may be associated with a reduced risk of contracting these conditions; students should be able to analyse, interpret and evaluate data associated with specific risk factors and the incidence of disease

How to approach it

Keep the two mechanisms in 3.2.1.1 distinct when explaining how a pathogen causes disease: cell damage (the direct destruction of host tissue by the pathogen) and toxin production (chemical substances the pathogen releases that disrupt normal cell or tissue function elsewhere in the body, sometimes far from the site of infection). Naming a specific pathogen and being able to say which of these two mechanisms, or both, it uses is a stronger answer than a general statement that “pathogens cause disease.”

Also fix the specific interfaces the specification names — digestive, reproductive and gas-exchange systems — as examples of the entry points where pathogens penetrate a mammal’s defences; the specification introduces these with “include”, so they are not the complete list (the skin is a further interface). Being able to name a specific interface relevant to a given disease example (for instance, the gas-exchange system for an airborne respiratory pathogen) demonstrates the applied understanding this sub-topic is testing, rather than a purely abstract definition.

For 3.2.1.2, this sub-topic is explicitly a data-handling skill as much as a content one — the specification’s own wording asks students to “analyse, interpret and evaluate data associated with specific risk factors and the incidence of disease.” Practise reading incidence data (for example, disease rates plotted against a risk factor such as smoking prevalence or dietary fat intake) and drawing a conclusion that distinguishes correlation from causation, since evaluating the strength and limitations of such evidence is what separates a top-band answer from a purely descriptive one.

Worked example: evaluating a lifestyle risk-factor claim

A question presents data showing a correlation between a country’s average dietary saturated fat intake and its rate of coronary heart disease, and asks candidates to evaluate what conclusions can be drawn.

Observation:  countries with higher average saturated fat intake tend
              to show higher rates of coronary heart disease in the
              data provided
Conclusion:   this is consistent with saturated fat intake being a risk
              factor for coronary heart disease
Limitation:   the data show correlation, not proof of causation; other
              variables that differ between countries (such as smoking
              rates, physical activity levels, or genetic factors) could
              also contribute to the differences in disease rate, so a
              single risk factor cannot be confirmed as the sole cause
              from this data alone

Explicitly separating what the data supports from what it does not prove is the specific skill 3.2.1.2 is built to assess.

Key terms to define precisely

Pathogen — an agent of infection, including bacteria and viruses, capable of causing disease in a host organism. Toxin — a poisonous chemical substance produced by a pathogen that disrupts normal cell or tissue function, sometimes at a site distant from the original infection. Interface — a boundary between an organism and its external environment, such as the digestive, reproductive or gas-exchange systems in a mammal, through which pathogens can gain entry. Risk factor — a variable statistically associated with an increased likelihood of developing a particular disease, without necessarily being proven to cause it directly. Correlation — a statistical relationship between two variables, which on its own does not establish that one variable causes the other; establishing causation requires additional evidence, such as a controlled experimental study or a clearly understood biological mechanism linking the two variables. Being able to explain, in these precise terms, why a correlation alone is insufficient evidence of causation is one of the most transferable analytical skills this sub-topic builds, and it recurs throughout the rest of Unit 2 wherever data on disease incidence is presented.

Common mistakes

Describing pathogens as causing disease only through direct damage, omitting toxin production as a separate mechanism. Failing to name a specific interface (digestive, reproductive, gas-exchange) when explaining how a pathogen enters the body. Treating a correlation between a lifestyle factor and disease incidence as proof of causation without acknowledging other contributing variables. Discussing risk factors for coronary heart disease and cancer as though they were identical, when specific risk factors differ between the two conditions.

Quick revision checklist

• Learn the two named mechanisms by which pathogens cause disease: cell damage and toxin production.
• Know the three interfaces the specification names as examples through which pathogens can enter a mammal’s body — digestive, reproductive and gas-exchange — and that this is not a complete list.
• Practise interpreting incidence data linked to a lifestyle risk factor, explicitly separating correlation from causation.
• Be ready to name at least one specific risk factor each for coronary heart disease and for cancer.

Related resources

• The Causes of Disease revision notes
• The Causes of Disease practice questions

Official syllabus

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