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C3.2.1—Pathogens cause infectious diseases

Pathogens cause infectious diseases explains how a specific barrier, pathogen interaction, immune mechanism or treatment response contributes to protection against infectious disease.

Syllabus
First assessment 2025
Objective
C3.2.1
Level
SL

Exam analysis

Chance of appearing5%of analysed past papers
Latest appearanceMay 2024
Most common paperPaper3
Typical marks1–3

Common command terms

  • Describe
  • Distinguish
  • Outline
  • Define
  • State
  • Explain

Scoring notes

Common mistake
Naming symptoms without linking them to the causative pathogen or transmission route.

Recent exam appearances

May 2024Paper3 ["SL"] · TZ117(b)[ 1 ]C3.2.1—Pathogens cause infectious diseases
May 2024Paper3 ["SL"] · TZ117(a)[ 3 ]C3.2.1—Pathogens cause infectious diseases
May 2024Paper3 ["SL"] · TZ216(a)[ 1 ]C3.2.1—Pathogens cause infectious diseases
May 2023Paper3 ["SL"] · TZ118(c)[ 3 ]C3.2.1—Pathogens cause infectious diseases
May 2023Paper3 ["SL"] · TZ217(b)[ 3 ]C3.2.1—Pathogens cause infectious diseases
Practice this objective

Coverage 2014–2024 · Updated 16 Jul 2026

Pathogens cause disease by entering and damaging a host

Pathogens cause disease by entering and damaging a host.

A pathogen is a disease-causing organism or particle. Bacteria, viruses, fungi and protists reproduce or use host cells in ways that disrupt tissues, release toxins or trigger damaging inflammation.

classify the pathogen; route of entry/transmission; host process disrupted; resulting signs.

A respiratory virus enters droplets, infects airway cells and spreads before the immune response clears it.

Pathogen presence is not identical to symptoms: dose, host susceptibility and immune response also affect disease.

Pathogens cause infectious diseases

Assessment in practice

2–3 marks
How it is assessed

This objective is assessed through structured response, commonly using Describe / Distinguish / Outline.

Command terms

Describe / Distinguish / Outline / Define / State / Explain

What earns marks

Mark schemes repeatedly reward named details, correct sequence and clear distinctions between similar processes.

Watch for

Naming symptoms without linking them to the causative pathogen or transmission route.

Representative question

Question 1

[Maximum number: 6]

Describe the cause, transmission and effects of malaria.

Defence Against Disease

  • Skin, mucus, cilia, lysozyme and clotting form primary barriers against pathogens.
  • Innate immunity is rapid and broad: phagocytes recognize, engulf and digest pathogens. Adaptive immunity is antigen-specific and forms memory.
  • Helper T-cells coordinate responses; activated B-cells undergo clonal selection, producing antibody-secreting plasma cells and memory cells. A second exposure therefore triggers a faster, stronger response.
  • HIV infects CD4 helper T-cells; their loss weakens immune coordination and can lead to AIDS.
  • Antibiotics target bacterial processes, not viruses. Antibiotic exposure selects resistant variants, which can spread by reproduction or plasmid transfer.
  • Vaccination creates active artificial immunity; high population immunity can indirectly protect susceptible people.
  • Evaluate disease and vaccine claims using reliable sources, trends, controlled comparisons, incidence and efficacy—not raw totals alone.

Concept essentials

  • Pathogens cause infectious diseases has to be linked to the exact pathogen, cell, molecule or population process involved.
  • The biological effect of pathogens cause infectious diseases depends on the sequence from trigger to protective outcome.
  • Clear distinctions within pathogens cause infectious diseases prevent confusion with neighbouring immune responses.
  • Evidence for pathogens cause infectious diseases is strongest when mechanism and consequence are explained together.